ARM: OMAP2+: hwmod: consolidate finding the MPU port index and storing it

An IP block's MPU interface port only needs to be found once.  The result
can be cached to speed further lookups.  This patch consolidates these
two steps into a single function.

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