drivers/flash/Kconfig.nrf_rram: bufferd write by default

Enable buffered write for nRF54l RRAM flash driver by default.
CONFIG_NRF_RRAM_WRITE_BUFFER_SIZE=1 means that write will be buffered
by 16 B buffer (native RRAM write-bock-size).
This allows optimal life-endurance of RRAM memory.

For reference" DTS declares 16 write-block-size already.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>