ARM: dts: omap4-droid4: Add gpio-keys

On Droid 4 "Volume Down" and "Keypad Slide" keys are
connected via SoC GPIOs, "Power Key" via CPCAP and
all other keys via SoC keypad Matrix. This adds the
GPIO keys.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
[tony@atomide.com: updated to apply on omap4-keypad patch]
Signed-off-by: Tony Lindgren <tony@atomide.com>