dt-bindings: Add dt bindings for silent boot mode

Power management Silent mode is a boot up mode with display and

audio disabled. This mode allows the platform to boot up the kernel

without enabling the UI, thereby masking the overall system boot latency.

Predominant usage could be seen in automotive applications where

boot latency is critical.

Top 2 use cases could be:

- One possible occasion for booting silently is when the driver approaches the

  vehicle and based on the trigger by proximity sensor the vehicle does a remote

  start before driver comes in the car.

- Another might be if the system needs to run in the middle of

  the night to perform updates.

Add " pm_silentmode_kernel_state"  viewable/editable via sysfs.

Kernel driver to monitor external GPIO input to indicate boot into

kernel silent mode or not. This driver sets up the GPIO and links it

up with an IRQ that is rising/falling edge triggered.

On the GPIO rising edge, the IRQ thread is used to clear the sysfs

in the PM framework that holds the state of the pm_silentmode_kernel_state.

On the GPIO falling edge, the IRQ thread is used to set the sysfs

in the PM framework that holds the state of the pm_silentmode_kernel_state.

The Device tree entry for silent mode can be used to register a GPIO

to the driver framework and use it to communicate to the Vehicle

interface processor.

Required properties:

- compatible: "qcom,silent-mode"

- qcom,silent-boot-gpio: GPIO needed HW interface from VIP to SoC

- interrupts: Assign interrupt that is dual edge triggered

- interrupt-names: Name of the interrupt under /proc/interrupt

Example:

	qcom,silent-mode {

		compatible = "qcom,silent-mode";

		qcom,silent-boot-gpio = <&pm8150_1_gpios 6 GPIO_ACTIVE_LOW>;

		interrupts = < 0xc5 0 IRQ_TYPE_EDGE_BOTH>;

		interrupt-names = "silent_pm8150_1_gpio6";

	};