arm64: QorIQ: gpio: return BUS_PROBE_DEFAULT

There is no reason this driver can't return default probe value.

Submitted by: Artur Rojek <ar@semihalf.com>
Reviewed by: emaste, mmel
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D26869