net: phy: Add generic helpers to access MMD PHY registers

Two new helper functions (phy_read_mmd() and phy_write_mmd()) are added
to allow access to the MMD PHY registers.

The MMD PHY registers can be accessed by several means:

1. Using two new MMD access function hooks in the PHY driver. These
functions can be implemented when the PHY driver does not support the
standard IEEE Compatible clause 45 access mechanism described in clause
22 or if the PHY uses its own non-standard access mechanism.

2. Direct access for C45 PHYs and C22 PHYs when accessing the reachable
DEVADs.

3. The standard clause 45 access extensions to the MMD registers through
the indirection registers (clause 22) in all the other cases.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>