ARM: dts: msm: Add mailbox configuration for SPSS on msmskunk

Add new entry in device tree to enable communication to SPSS with
the SPSS mailbox transport.

CRs-Fixed: 1059650
Change-Id: I382c2a3f31c1760b0e33dcb345243c83dd5b790a
Signed-off-by: Chris Lew <clew@codeaurora.org>