Input: mc13783-pwrbutton - convert to use module_platform_driver()

This patch converts mc13783-pwrbutton to use the module_platform_driver()
macro which makes the code smaller and a bit simpler.

Also staticise mc13783_pwrbutton_driver which is not used outside
this driver so no need to make the symbol global.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>