PCI: armada8k: Add support for gpio controlled reset signal

Add support for the gpio reset signal binding as described in the
designware-pcie.txt DT binding document. Both the documented
'reset-gpio' property name and the more standard 'reset-gpios' name are
supported.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>