drivers: mb85rc: support use of multiple modules as a single one

Allow use of multiple mb85rc frams at contiguous i2c addresses as a single
big fram module.
Tested on mb85rc1mt used as two 32K modules, where the first one was at
mb85rc1mt's first i2c address and the second one at mb85rc1mt's second i2c
address.

Signed-off-by: Jakub Michalski <jmichalski@internships.antmicro.com>
Signed-off-by: Mateusz Sierszulski <msierszulski@antmicro.com>