arm: process.c: Define arm_pm_idle

Define arm_pm_idle to be arch_idle which is defined in pm-8x60.c
(This code should be moved to the pm-8x60 file later on)

Change-Id: I345d436111c585e4dfbae367dac5d24d1a1f9711
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>