ARM: dts: imx6qdl-dhcom-pdk2: Include missing headers

Include missing headers, since this DT source uses symbolic names defined
in those headers. If an external DT includes this dtsi without including
the headers as well, the DT becomes unbuildable.

Three headers are included:
dt-bindings/gpio/gpio.h -- because this DT contains GPIO_ACTIVE_HIGH
dt-bindings/input/input.h -- because this DT contains KEY_A
dt-bindings/pwm/pwm.h -- because this DT contains PWM_POLARITY_INVERTED

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: kernel@dh-electronics.com
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>