ARM: dts: DRA7: Add node for RTC

Add node for RTC.

Note that on dra7xx are no separate interrupts for alram
and timer unlike for SoCs.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[nm@ti.com: update with rtc crossbar number]
Signed-off-by: Nishanth Menon <nm@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>