hwrng: Kconfig - modify default state for atmel-rng driver

In order to prevent building the Atmel hw_random driver for each and every
configuration, add a "default" Kconfig state in relation with
CONFIG_ARCH_AT91.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>