powerpc/fsl: Make fsl_deep_sleep() usable w/ modules and non-83xx builds

Export is needed for modular builds, and a static inline stub is needed
for non-MPC83xx builds.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>