apm821xx: consolidate apm821xx device trees files

This patch moves the common SoC device tree entries
from the currently four supported platforms into a
common apm82181.dtsi.

Furthermore, this patch also changes the GPIO, IRQ and
input definitions of the supported platforms to use the
defined dt-bindings macros for GPIO_ACTIVE_LOW|HIGH,
KEY_WPS|RESTART|*, IRQ_TYPE_* when it's appropriate.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>