mips: mscc: DT: Update luton device tree to use fast SPI driver

Thes patch change the luton base device tree to use the newly added
SPI bitbang driver.

It also updates the "mscc_luton_defconfig" to use the new driver.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>