ARM: dts: am57xx-evm: Add 'gpios' property with gpio2_8

gpio2_8 is connected to the PCIe_RESETn line and it has to be driven low to
reset the PCIe cards.  Add gpios property to PCIe DT node.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Tony Lindgren <tony@atomide.com>