The power supplies to PCIe port are often controlled by GPIO on some board
designs.  Let's add an optional regulator which can be backed by GPIO to
control the power.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: NRob Herring <robh@kernel.org>