staging: mt7621-pinctrl: replace core_initcall_sync with builtin_platform_driver

We don't really need initialization of this at such an early stage.
Just use builtin_platform_driver to initialize this driver.

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