ARM64: juno: add GPIO keys

ARM64: juno: add GPIO keys

The Juno board has two keys connected to a PL061 GPIO block,
in accordance to DDI0524B "ARM Versatile Express Juno Development
Platform" revision 1.0, table 2-4 "GPIO (0) and GPIO (1) used
for additional user key entry". By trial-and-error I found that
these are connected to the two keys named "power" and "home"
on the motherboard.

Register the GPIO block and these two keys in the device tree
using the PL061 GPIO driver and the generic gpio keys.

- Map POWER, HOME, VOL+ and VOL- to the obvious input events.
- Map RLOCK to KEY_SCREENLOCK/KEY_COFFEE unless someone can
  explain better what this is for.
- Map the NMI button to KEY_SYSREQ as this is used like so
  in the SYSREQ debugging hack.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>