* ARMv7M System Timer

ARMv7-M includes a system timer, known as SysTick. Current driver only
implements the clocksource feature.

Required properties:
- compatible	  : Should be "arm,armv7m-systick"
- reg		  : The address range of the timer

Required clocking property, have to be one of:
- clocks	  : The input clock of the timer
- clock-frequency : The rate in HZ in input of the ARM SysTick

Examples:

systick: timer@e000e010 {
	compatible = "arm,armv7m-systick";
	reg = <0xe000e010 0x10>;
	clocks = <&clk_systick>;
};

systick: timer@e000e010 {
	compatible = "arm,armv7m-systick";
	reg = <0xe000e010 0x10>;
	clock-frequency = <90000000>;
};