Spreadtrum mcd bindings

Required properties:
- compatible: Should be one of the following:
  "sprd,roc1-modem-ctrl",
  "sprd,orca-modem-ctrl".
- poweron-gpio: control modem power.
- reset-gpio: control modem reset.
- cpwatchdog-gpio: modem watchdog reset int. It should be set as input and high level trigger for AP
  subsystem, and set as output and high level trigger for CP subsystem as defaut configuration.
- cpassert-gpio: modem assert int. Its default configuration should be same as cpwatchdog-gpio.
- cppanic-gpio: modem panic int. Its default configuration should be same as cpwatchdog-gpio.
- cppoweroff-gpio: let modem poweroff int. Its default configuration should be same as cpwatchdog-gpio.
Example:

	sprd-mcd {
		compatible = "sprd,roc1-modem-ctrl";
		poweron-gpio = <&ap_gpio 15 GPIO_ACTIVE_LOW>;
		reset-gpio = <&ap_gpio 16 GPIO_ACTIVE_LOW>;
		cpwatchdog-gpio = <&ap_gpio 30 GPIO_ACTIVE_LOW>;
		cpassert-gpio = <&ap_gpio 36 GPIO_ACTIVE_LOW>;
		cppanic-gpio = <&ap_gpio 31 GPIO_ACTIVE_LOW>;
		cppoweroff-gpio = <&ap_gpio 33 GPIO_ACTIVE_HIGH>;
	};
