mmc: MAINTAINERS update for omap_hsmmc

Update Maintainer email for omap_hsmmc, as Venkatraman will no longer
be able to maintain omap_hsmmc driver.

Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by: Venkatraman S <svenkatr@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>