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

Add RPMSG_GET_SIGNAL_IOCTL and RPMSG_SET_SIGNAL_IOCTL 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>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/1681807721-32343-4-git-send-email-quic_sarannya@quicinc.com