net: phy: adin: add {write,read}_mmd hooks

Both ADIN1200 & ADIN1300 support Clause 45 access for some registers.
The Extended Management Interface (EMI) registers are accessible via both
Clause 45 (at register MDIO_MMD_VEND1) and using Clause 22.

The Clause 22 access for MMD regs differs from the standard one defined by
802.3. The ADIN PHYs  use registers ExtRegPtr (0x0010) and ExtRegData
(0x0011) to access Clause 45 & EMI registers.

The indirect access is done via the following mechanism (for both R/W):
1. Write the address of the register in the ExtRegPtr
2. Read/write the value of the register via reg ExtRegData

This mechanism is needed to manage configuration of chip settings and to
access EEE registers via Clause 22.

Since Clause 45 access will likely never be used, it is not implemented via
this hook.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>