ARM: OMAP2+: hwmod: add _find_mpu_rt_port()

Most IP blocks on the OMAP SoC have an interconnect link that is
intended to be used by the MPU to communicate with the IP block.
Several parts of the hwmod code need to be able to identify this link.
Currently, this is open-coded.  However, future patches will change
the way that interconnect links are represented and will make
identifying the link more complex.  So to avoid code duplication, this
patch centralizes the MPU port link identification code into a new
function, _find_mpu_rt_port().

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