hwrng: Fix a few driver dependencies and defaults

hwrng: Fix a few driver dependencies and defaults

HW_RANDOM_ATMEL should not only default to ARCH_AT91, it should depend
on it. This driver is useless on other architectures.

Likewise, HW_RANDOM_EXYNOS should depend on ARCH_EXYNOS, as it it
useless on other architectures.

Also set the default to HW_RANDOM for the few architecture-specific
drivers which didn't have it yet, for consistency.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>