hwrng: mpfs - add polarfire soc hwrng support

Add a driver to access the hardware random number generator on the
Polarfire SoC. The hwrng can only be accessed via the system controller,
so use the mailbox interface the system controller exposes to access the
hwrng.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>