mfd: vexpress: Give the "leds-gpio" device a real parent

The registration of the "leds-gpio" device was using
"vexpress_sysreg_dev" as a parent before it was actually
set to something different than NULL.

Trivial fix by reordering the code.

Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>