drivers/flash/rram: set to use up to 512 B burst writes by default

Now RRAMC is set to use up to 512 B burst writes by default as most
time effective.
Requested slot time was changed to 8000 us for that case, as this is
required in order to hold the longest write operation.
Drivers differs slot duration depending on configured RRAMC buffers count
(CONFIG_NRF_RRAM_WRITE_BUFFER_SIZE).

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