mfd: vexpress: Add pseudo-GPIO based LEDs

The LEDs on the Versatile Express motherboard are controlled
through simple memory-mapped register. This patch extends
the pseudo-GPIO controller definition for these lines and
creates generic "leds-gpio" device using them

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>