net: stmmac: use integrated PCS for BASE-X modes

net: stmmac: use integrated PCS for BASE-X modes

dwmac-qcom-ethqos supports SGMII and 2500BASE-X using the integrated
PCS, so we need to expand the PCS support to include support for
BASE-X modes.

Add support to the prereset configuration to detect 2500BASE-X, and
arrange for stmmac_mac_select_pcs() to return the integrated PCS if
its supported_interfaces bitmap reports support for the interface mode.

This results in priv->hw->pcs now being write-only, so remove it.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w2tPj-0000000DYAv-2JcZ@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>