nvmem: eeprom: at25: fram discovery simplification

Changed "is_fram" to bool and set it based on compatible string.

Signed-off-by: Jiri Prchal <jiri.prchal@aksignal.cz>
Link: https://lore.kernel.org/r/20210611152416.68386-1-jiri.prchal@aksignal.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>