ARM: dts: omap4-droid4: Add GPIO poweroff

Droid 4 has a GPIO line that we can use with CONFIG_POWER_RESET_GPIO.
It is probably connected to the CPCAP PMIC, and seems to power down
the whole device taking power consumption to zero based on what
I measured.

Cc: devicetree@vger.kernel.org
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Tested-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>