staging: mt7621-gpio: make use of 'builtin_platform_driver'

This driver was being registered using 'module_platform_driver'
but it is not a module at all. Instead of this use
'builtin_platform_driver' which seems to be the correct one.

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