arm64: QorIQ: gpio: Style cleanups

Make the code more conformant to style(9) and improve the general
readability.

This patch does not alter the driver logic.

Submitted by: Artur Rojek <ar@semihalf.com>
Reviewed by: mmel
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D26867