msm: board-8974: Call of_mpm_init function

Add a call to of_mpm_init function from within the msm_8974_init_irq().
The MPM drivers keeps account of all enabled interrupts in the system to
be able to configure the MPM hardware during low power sleep modes. To
ensure that MPM receives notification for all interrupts, it needs to be
initialized as soon as the system interrupt controllers are initliazed.

Change-Id: I61f01fc9a7a6caf5b950f24cf7474f86c97745ca
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>