exynos: Cleanup exynos_init

- None of the callers perform error checking and based on the non-empty
  versions of this function, there's no checking to be done, so make
  this a void.
- Add a default weak version of the function.
- Remove the empty versions of exynos_init now that we have a weak
  version.

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