mmc: omap: convert to module_platform_driver

Get rid of boilerplate code by using module_platform_driver macro,
no functional changes.

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