rpmsg: char: Add RPMSG GET/SET FLOWCONTROL IOCTL support

Add RPMSG_GET_OUTGOING_FLOWCONTROL and RPMSG_SET_INCOMING_FLOWCONTROL
IOCTL support for rpmsg char device nodes to get/set the low level
transport signals.

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/1688679698-31274-4-git-send-email-quic_sarannya@quicinc.com


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