ARM: dts: doc: Document missing binding for omap5-mpu

The binding and support for omap5-mpu which has a cortex-a15
smp core, gic and integrated L2 cache has been existing for sometime.
So Documenting the missing binding here.

Cc: Benoit Cousson <bcousson@baylibre.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>