Looking at the SGMII PCS from SJA1110, which is accessed indirectly
through a different base address as can be seen in the next patch, it
appears odd that the address accessed through indirection still
references the base address from the SJA1105S register map (first MDIO
register is at 0x1f0000), when it could index the SGMII registers
starting from zero.

Except that the 0x1f0000 is not a base address at all, it seems. It is
0x1f << 16 | 0x0000, and 0x1f is coding for the vendor-specific MMD2.
So, it turns out, the Synopsys PCS implements all its registers inside
the vendor-specific MMDs 1 and 2 (0x1e and 0x1f). This explains why the
PCS has no overlaps (for the other MMDs) with other register regions of
the switch (because no other MMDs are implemented).

Change the code to remove the SGMII "base address" and explicitly encode
the MMD for reads/writes. This will become necessary for SJA1110 support.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>