realtek: RTL839x: reorganize mdio functions and SerDes register layout

realtek: RTL839x: reorganize mdio functions and SerDes register layout

The RTL839x mdio functions are scattered around the code. Relocate
them to the bus (still inside the ethernet driver).

Additionally provide a consistent SerDes register access through the
mdio bus. Until now when a SerDes directly drives a SFP module there
is no clear rule of how to handle its register set that consists of
two parts:

- c22 phy registers 0-15 live in the fiber page (2) of the SerDes
- other SerDes specific registers exist in pages before and after

The mdio bus and other SerDes functions are a wild mix of directly
looking into page 2 or just using self defined methods to access
data.

Adapt the bus to the new consistent phy interface that mixes the
SerDes register set like classic Realtek phys do it.

- Use register 31 as page select (already in the bus)
- Always keep the common registers 0-15 in place and read fiber page
- Map the SerDes internal registers into the upper vendor specific
  registers 16-23 according to the page select register (31).

That gives a register mapping as follows:

+-----------------------+-----------------------+---------------+-------------+
| reg 0x00-0x0f         | reg 0x10-0x17         | reg 0x18-0x1e | reg 0x1f    |
+-----------------------+-----------------------+---------------+-------------+
| SerDes fiber page (3) | real SerDes registers | zero          | SerDes page |
| registers 0 - 15      | in packages of 8      |               | select reg  |
+-----------------------+-----------------------+---------------+-------------+

Example to make it as clear as possible.

SerDes registers on a RTL839x show

Page / Reg   | 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0A 0x0B ...
-------------+----------------------------------------------------------------
0 - SDS      | 0C03 0F00 7060 7106 074D 0EBF 0F0F 0359 5248 0000 0F80 0000 ...
1 - SDS_EXT  | 0000 0000 85FA 8C6D 5CCC 0000 20D8 0003 79AA 8C64 00C3 1482 ...
2 - FIB      | 1140 6189 001C CA40 01A0 0000 0000 0004 0000 0000 0000 0000 ...
3 - FIB_EXT  | 1140 6109 001C CA40 01A0 0000 0000 0004 0000 0000 0000 0000 ...

This translates to this phy layout

             | SerDes fiber registers  normal SerDes registers  zero     p.sel
Page / Reg   | 0x00 0x01 0x02 0x03 ... 0x10 0x11 0x12 0x13 ...  0x18 ... 0x1f
-------------+---------------------------------------------------------------
0            | 1140 6189 001C CA40 ... 0C03 0F00 7060 7106 ...  0000 ... 0000
1            | 1140 6189 001C CA40 ... 5248 0000 0F80 0000 ...  0000 ... 0001
...
4            | 1140 6189 001C CA40 ... 0000 0000 85FA 8C6D ...  0000 ... 0004
...

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19634
Signed-off-by: Robert Marko <robimarko@gmail.com>