msm: mpm: cleanup the calls to of_mpm_init

The of_mpm_init function expects the board file to find its node.
This can be done in a function within the mpm driver. Cleanup
the code.

Change-Id: Ic1449da203aabdf5e8b87a0141cfec6345b00798
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>