hwrng: pic32 - Add PIC32 RNG hardware driver

Add support for the hardware true random number generator
peripheral found on PIC32.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>