dt-bindings: clk: add AOP clock controller bindings

The Always on Processor (AOP) found on Qualcomm Technologies, Inc.
SoCs supports enabling, disabling, and scaling several shared
clocks.  These clocks are controlled using the QMP mailbox
protocol.  Add device bindings for AOP clock controllers.

Change-Id: I1d0b6c0abad702b2bd50fd2078eedc0907b85214