dt-bindings: mfd: qcom,spmi-pmic: add documentation for qcom,can-sleep

Add documentation for the qcom,can-sleep property.  This is used
for slow SPMI busses which may sleep during transactions.

Change-Id: Ib07ffa28a0aa167571501e2493f5f03ad575755e
Signed-off-by: David Collins <collinsd@codeaurora.org>