OMAP2+: hwmod: Mark functions used only during initialization with __init

_register, _find_mpu_port_index and _find_mpu_rt_base are static APIs
that will be used only during the omap_hwmod initialization phase.
There is no need to keep them for runtime.

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