ARM: mvebu: x530: set MPP55 to gpio

MPP55 is used as a reset connected to the L3 switch chip. This doesn't
matter for u-boot as it doesn't use the L3 switch but it is useful to
be able to toggle the switch in/out of reset for the OS.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>