Wakeup with a DMTIMER on am335x boards

This driver should add additonal support for wakeup from dmtimer on am335x based boards. This can be useful if the RTC on the chip is not powered/enabled and therefore it is not possible to have a timer based wakeup source from standby.

Only tested with kernel 4.4.71 with patches from ti SDK. The mainline kernel does not support standby for am335x at the moment.

Add the following entry to the devicetree file. Only standby (Deepsleep 1) is supported for now, Deepsleep 0 does not work, because dmtimer0 and dmtimer1 are already used internally.

wakeup_timer0: wakeup_timer@0 {
	status = "okay";
	compatible = "wakeup-timer";
	timer = <&timer3>;
	interrupts = <68>;	 // To be verified
	interrupt-names	= "wakeup";	// To be verified
	wakeup-source;	// To be verified
};

&timer3 {
		status = "okay";
		wakeup-source;	// To be verified
		ti,no-idle;
};