RAVE SP firmware versions found on RDU2 provide decicated bootsource
manipulation commands, which, under the hood, are just thin wrappers
around write/read to a particular location in EEPROM.

Unfortunately, RAVE SP firmware found on RDU1, does not provide any
dedicated bootsource commands and its users are expected to access
EEPROM directly.

In order to avoid having separate code paths for handling those two
cases, convert the code to access EEPROM directly via NVMEM API.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>