MAINTAINERS: Add Aaro Koskinen as TI omap1 SoC co-maintainer

Aaro has been doing a great job making sure mach-omap1 stays working
with the mainline kernel. So let's add Aaro as omap1 co-maintainer to
the MAINTAINERS file.

Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>