Revert "staging: mt7621-pinctrl: replace core_initcall_sync with builtin_platform_driver"

That patch causes the network interface on the device to stop working.
device_initcall() is called much later than core_initcall_sync() and
that seem to be a problem. Revert it to get a correct behaviour.

Reported-by: NeilBrown <neil@brown.name>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>