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>

MAINTAINERS

@@ -7124,6 +7124,7 @@ F:	drivers/clk/spear/
 
 SPI SUBSYSTEM
 M:	Grant Likely <grant.likely@secretlab.ca>
+M:	Mark Brown <broonie@opensource.wolfsonmicro.com>
 L:	spi-devel-general@lists.sourceforge.net
 Q:	http://patchwork.kernel.org/project/spi-devel-general/list/
 T:	git git://git.secretlab.ca/git/linux-2.6.git