MAINTAINERS: add OMAP defconfigs under OMAP SUPPORT

omap2plus_defconfig and omap1_defconfig are also
part of the OMAP Support maintained, because of
that it's best to list them under OMAP SUPPORT on
MAINTAINERS so people know to Cc linux-omap when
patching them.

Reported-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>