linux-phytec-ti

MAINTAINERS: Add myself as a backup maintainer for SPI

Grant said he would find it helpful for me to continue handling some of
the legwork for SPI so add myself to MAINTAINERS so I get CCed on
patches.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>