ARM: DRA7: board-generic: Add basic DT support

Describe minimal DT boot machine details for DRA7xx based SoC's. DRA7xx
family is based on dual core ARM CORTEX A15 using GIC as the interrupt controller.
The PRCM and timer infrastructure is reused from OMAP5 and so are the io
descriptor tables.

Signed-off-by: R Sricharan <r.sricharan@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>