PCI: imx6: Allow probe deferral by reset GPIO

Some designs implement reset GPIO via a GPIO expander connected to a
peripheral bus.  One such example would be i.MX7 Sabre board where said
GPIO is provided by SPI shift register connected to a bitbanged SPI bus.
To support such designs, allow reset GPIO request to defer probing of the
driver.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: yurovsky@gmail.com
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Dong Aisheng <dongas86@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org