PCI: mvebu: Use gpio_desc to carry around gpio

Use a gpio_desc to carry around the gpio, so we can then make use of the
GPIOF_ACTIVE_LOW property rather than carrying that around as well.  This
also avoids needing to use gpio_is_valid() to check whether we have a GPIO;
checking for a non-NULL descriptor is simpler.

Tested-by: Willy Tarreau <w@1wt.eu> (Iomega iConnect Kirkwood, MiraBox Armada 370)
Tested-by: Andrew Lunn <andrew@lunn.ch> (D-Link DIR664 Kirkwood)
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> (Armada XP GP)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>