net: stmmac: convert to phylink PCS support

net: stmmac: convert to phylink PCS support

Now that stmmac's PCS support is much more simple - just a matter of
configuring the control register - the basic conversion to phylink PCS
support becomes straight forward.

Create the infrastructure to setup a phylink_pcs instance for the
integrated PCS:
- add a struct stmmac_pcs to encapsulate the phylink_pcs structure,
  pointer to stmmac_priv, and the core-specific base address of the PCS
  registers.
- modify stmmac_priv and stmmac_mac_select_pcs() to return the embedded
  phylink_pcs structure when setup and STMMAC_PCS_SGMII is in use, and
  move the comment from stmmac_hw_setup() to here.
- create stmmac_pcs.c, which contains the phylink_pcs_ops structure, a
  dummy .pcs_get_state() method which always reports link-down, and
  .pcs_config() method, moving the call to stmmac_pcs_ctrl_ane() here,
  but without indirecting through the dwmac specific core code. The
  link-down behaviour mentioned above maintains the current behaviour
  when phylink is used with inband but without a PCS.

This will ensure that the PCS control register is configured to the
same settings as before, but will now happen when the netdev is opened
or reusmed rather than only during probe time. However, this will be
before the .fix_mac_speed() method is called, which is critical for the
behaviour in dwmac-qcom-ethqos's ethqos_configure_sgmii() function to
be maintained.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/E1v9P72-0000000AomR-3ro4@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>