Some boards do not have a PCIe reset GPIO.  To avoid probe failure on these
boards, make the reset GPIO optional as well.

[bhelgaas: whitespace fixes]
Signed-off-by: NMarek Vasut <marex@denx.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NJingoo Han <jg1.han@samsung.com>
Acked-by: NShawn Guo <shawn.guo@linaro.org>
Cc: Frank Li <lznuaa@gmail.com>
Cc: Harro Haan <hrhaan@gmail.com>
Cc: Mohit KUMAR <Mohit.KUMAR@st.com>
Cc: Pratyush Anand <pratyush.anand@st.com>
Cc: Richard Zhu <r65037@freescale.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Sean Cross <xobs@kosagi.com>
Cc: Siva Reddy Kallam <siva.kallam@samsung.com>
Cc: Srikanth T Shivanand <ts.srikanth@samsung.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Yinghai Lu <yinghai@kernel.org>