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>
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