net: stmmac: move most PCS register definitions to stmmac_pcs.c

net: stmmac: move most PCS register definitions to stmmac_pcs.c

Move most of the PCS register offset definitions to stmmac_pcs.c.
Since stmmac_pcs.c only ever passes zero into the register offset
macros, remove that ability, making them simple constant integer
definitions.

Add appropriate descriptions of the registers, pointing out their
similarity with their IEEE 802.3 counterparts. Make use of the
BMSR definitions for the GMAC_AN_STATUS register and remove the
driver private versions.

Note that BMSR_LSTATUS is non-low-latching, unlike it's 802.3z
counterpart.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Link: https://patch.msgid.link/E1vlmOV-00000006zv5-1CwO@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>