net: phy: realtek: Add functions to read PHY's extended registers

According to the datasheet to access the extended registers we have to:

1. Write Register 31 Data = 0x0XYZ (Page 0xXYZ)
2. Read/Write the target Register Data
3. Write Register 31 Data = 0x0000 or 0xa42 (switch back to IEEE
   Standard Registers)

Hook the missing functions so that we can use the `mdio rx/wx` command to
easily access the extended registers.

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