rpmsg: glink: Add support to handle signals command

Remote peripherals send signal notifications over glink with commandID 15.

Add support to send and receive the signal command and based signals
enable or disable flow control with remote host.

Signed-off-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Deepak Kumar Singh <quic_deesin@quicinc.com>
Signed-off-by: Sarannya S <quic_sarannya@quicinc.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/1688679698-31274-3-git-send-email-quic_sarannya@quicinc.com


Signed-off-by: Bjorn Andersson <andersson@kernel.org>