msm: pm: Remove redundant cpu_init API

cpu_init() API setup the per-CPU stacks after cores
comes out of the low power modes. This API is not
needed after the PM driver start using the standard
ARM suspend/resume framework.

Change-Id: Ic384165e4fbc289c8338c66ebac1bb0d2bfadf8b
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>