dt/bindings: Add bindings for the PIC32 random number generator

Document the devicetree bindings for the random number generator found
on Microchip PIC32 class devices.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>