Atmel AT91SAM9260 Real Time Timer

Required properties:
- compatible: should be one of the following:
	- "atmel,at91sam9260-rtt"
	- "microchip,sam9x60-rtt", "atmel,at91sam9260-rtt"
- reg: should encode the memory region of the RTT controller
- interrupts: rtt alarm/event interrupt
- clocks: should contain the 32 KHz slow clk that will drive the RTT block.
- atmel,rtt-rtc-time-reg: should encode the GPBR register used to store
	the time base when the RTT is used as an RTC.
	The first cell should point to the GPBR node and the second one
	encode the offset within the GPBR block (or in other words, the
	GPBR register used to store the time base).


Example:

rtt@fffffd20 {
	compatible = "atmel,at91sam9260-rtt";
	reg = <0xfffffd20 0x10>;
	interrupts = <1 4 7>;
	clocks = <&clk32k>;
	atmel,rtt-rtc-time-reg = <&gpbr 0x0>;
};