In preparation for NVMEM support, split get_of_eeprom() in subfunction
to tidy the code and facilitate the addition of alternative method to
get eeprom data. No behaviour change intended.

While at it also drop OF ifdef checks as OF have stubs and calling
of_get_property would result in the same error returned.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

Add support for providing eeprom in nvmem cells by adding nvmem cell as
an alternative source for mt76_get_of_eeprom().

Nvmem cells will follow standard nvmem cell definition and needs to be
called eeprom' to be correctly identified.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>