rng: Tighten some rng driver dependencies

rng: Tighten some rng driver dependencies

In this case, the NPCM RNG driver also provides some of the functions
that are implemented in the generic LIB_RAND file, so only allow this to
be built when that is disabled. The exynos RNG driver depends on ARM
SMCCC calls and so cannot be built outside of that. Express these
requirements in Kconfig as well.

Signed-off-by: Tom Rini <trini@konsulko.com>