samples: eeprom: Fix build issues with x_nucleo_eeprma2 sample

There are a number of platforms that the x_nucleo_eeprma2 eeprom sample
can't build on.  Also fix issue with missing include of <mem.h> header
to get DT_SIZE_* macros used by the overlay.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>