hwrng: imx-rngc - use BIT(x) for register bit defines

Rewrite the defines for register bits to use BIT(x) instead of writing
out the 32-bit number. This makes it easier to compare the code with the
register descriptions in the reference manual.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>