dt-bindings: qpnp-qg: Add properties to specify S2 config in sleep

Allow configuring the FIFO length, accumulator length and
accumulator interval when system goes to sleep during
discharge.

Change-Id: I8018f8033d05273320fd92cc7d21fcdce0885df6
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>