mfd: Register db8500-prcmu as a platform driver instead of only probing

Pass the probe function as part of the platform_driver struct and
register using the more common platform_driver_register call. In
subsequent patches we'll also add DT support into the struct.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>