msm: idle-v7: Write Krait-specific bits before WFI

Certain variants of the Krait processor require a
modification of an implementation-defined register prior to
executing the WFI instruction.

Change-Id: I6f0ff0b7622f00003d6e80e04514883d75a715a2
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>