msm: gsi: add support for NOP xfers

To allow more efficient event ring completion, a NOP (No Operation)
transfer is introduced. The client can use this new descriptor type to
aggregate completion to transfer for TO_GSI based on some timer. When
client sends a NOP transfer to GSI with IEOT (End of transfer indication),
GSI will generate an event when this NOP descriptor was processed.

Change-Id: I3a47bef5c02fec5c63bb8c3e7e4fe8b44fa6986e
CRs-Fixed: 2022853
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>