drivers: spi: Kconfig.nrfx: Use RAM buffers for SPIMs by default

Enable by default the use of RAM buffers in the spi_nrfx_spim.c
driver for copying TX data located in flash (as SPIM peripherals
cannot transfer directly form flash). Without this patch, users can
get confused, especially when SPI transaction is used by an upper
level driver which does not check all error codes.
For size of the buffer, use the value used so far in the reel_board
default configuration and in the SPI loopback test, i.e. 8 bytes.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>