drivers/flash/nrf_qspi_nor: Default write_from_nvmc buffer size to 4

The commit changes the default size of write_from_nvmc,
defined by CONFIG_NORDIC_QSPI_NOR_STACK_WRITE_BUFFER_SIZE, to 4,
making the write_from_nvmc operation enabled by default.
The Kconfig description for the option has been changes more clearly
describe how does the option impact compilation.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>