power: smb135x-charger: Add suspend/resume callbacks

To avoid unnecessary wakeups from the SMB chip during system
suspend, only enable important interrupts while entering
into suspend.

Save and restore the intially configured IRQs in suspend
and resume respectively.

Change-Id: I295f594ce0f08f1552149a9b0f6cc40521495a49
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>