hwrng: bcm63xx - move register definitions to driver

arch/mips/include/asm/mach-bcm63xx/bcm63xx_regs.h contains the register
definitions for this random number generator block, incorporate these
register definitions directly into the bcm63xx-rng driver so we do not
rely on this header to be provided.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>