MAINTAINERS: add spi support to GXP

Add the spi driver and dt-binding documentation

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Link: https://lore.kernel.org/r/20220728161459.7738-6-nick.hawkins@hpe.com
Signed-off-by: Mark Brown <broonie@kernel.org>