drivers/rng: simplify Kconfig

For all sandbox systems with DM_RNG we enable RNG_SANDBOX. So we can simply
set the default to yes.

All rng drivers depend on DM_RNG. Use a single 'if' instead of individual
dependencies. Now 'make menuconfig' shows the individual drivers neatly
indented under the DM_RNG entry.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sughosh Ganu <sughosh.ganu@linaro.org>