hwrng: Turn HW_RANDOM into a menuconfig

This makes configuration more convenient IMHO, and avoids having to
repeat the dependency on HW_RANDOM for every single driver.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>