ARM: dts: DRA7: add entry for qspi mmap region

Add qspi memory mapped region entries for DRA7xx based SoCs. Also,
update the binding documents for the controller to document this change.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>