ramips: mt7621: convert MT7915 EEPROM to NVMEM format

ramips: mt7621: convert MT7915 EEPROM to NVMEM format

Some MT7915 calibration data consists of two parts. The first part
"eeprom" size is 0xe00. The second part "precal" size is 0x19c10.

Though some devices may not have precal data, it's better to assume
that precal data exists as no users/developers confirm it. On the
other hand, some devices definitely do not contain precal data
because the EEPROM partition size is smaller than the precal NVMEM
cell size.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>