hwrng: ba431 - add support for BA431 hwrng

Silex insight BA431 is an IP designed to generate random numbers that
can be integrated in various FPGA.
This driver adds support for it through the hwrng interface.

This driver is used in Silex Insight Viper OEM boards.

Signed-off-by: Olivier Sobrie <olivier.sobrie@silexinsight.com>
Signed-off-by: Waleed Ziad <waleed94ziad@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>