Modem chipset attached to Application processor

Modem chipset can be connected to an external apss processor. The control
channel between the two chipsets consists of gpios that can relay the
state of one subsytem to another. Modem can indicate different events
(bootup/crash etc.) to AP and can get the same information from AP.

Required Properties:
- compatible:	"qcom,sdx-ext-ipc".

Required named gpio properties:
- qcom,mdm2ap-status-gpio: gpio for modem to indicate the boot status to APQ.

- qcom,ap2mdm-status-gpio: gpio for APQ to indicate the boot status to modem.



Optional named gpio properties:
- qcom,mdm2ap-status2-gpio: gpio for modem to indicate to APQ that it is in
		E911 call or doing firmware upgrade.

- qcom,ap2mdm-status2-gpio: gpio for APQ to indicate graceful shutdown to modem.

Example:
	sdx_ext_ipc: qcom,sdx_ext_ipc {
		compatible = "qcom,sdx-ext-ipc";
		qcom,ap2mdm-status-gpio = <&tlmm 64 0x00>;
		qcom,ap2mdm-status2-gpio = <&tlmm 65 0x00>;
		qcom,mdm2ap-status-gpio = <&tlmm 63 0x00>;
		qcom,mdm2ap-status2-gpio = <&tlmm 66 0x00>;
	};