mmc: dw_mmc: use helper macro module_platform_driver()

Since v3.2 we have nice macro to define the platform driver's init and exit
calls. This patch simplifies the dw_mmc platform driver by using that macro.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Girish K S <girish.shivanajappa@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>