rpmsg: glink: spss: Add pipe reset handlers

SPSS assumes the data in the spss fifo descriptors is correct if the
mailbox registers are initialized. Add handlers to reset the pipe for
the SSR case.

Change-Id: I4eb76018b8be934fc96857dcf2ee62e3c7b73e95
Signed-off-by: Chris Lew <clew@codeaurora.org>