OMAP2+: hwmod: Make omap_hwmod_register private and remove omap_hwmod_unregister

Do not allow omap_hwmod_register to be used outside the core
hwmod code. An omap_hwmod should be registered only at init time.
Remove the omap_hwmod_unregister that is not used today since the
hwmod list will be built once at init time and never be modified
at runtime.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>