OMAP2+: hwmod: find MPU initiator hwmod during in _register()

Move the code that looks for the MPU initiator hwmod to run during
the individual hwmod _register() function.  (Previously, it ran after
all hwmods were registered in the omap_hwmod_late_init() function.)

This is done so code can late-initialize a few individual hwmods --
for example, for the system timer -- before the entire set of hwmods is
initialized later in boot via omap_hwmod_late_init().

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