hwrng: Kconfig - Add HAS_IOMEM dependencies for exynos/meson/mtk/npcm

Add missing dependencies on HAS_IOMEM as otherwise they will trigger
failed builds with COMPILE_TEST enabled.

Also add dependencies on OF where appropriate.

Change the default so that these drivers are not enabled just because
COMPILE_TEST is turned on.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304191106.swKbBeDh-lkp@intel.com/


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>