soc: qcom: bgrsb: enable RSB to send command

bg-rsb driver makes use of bgrsb-rpmsg driver
for Glink communication.
This change enables rsb driver to send tx message
on RSB_CTRL channel with the help of rpmsg driver.

Change-Id: Ib68df84d88c42bd310c1a3676be61d991d711ba3
Signed-off-by: Protik Biswas <protbisw@codeaurora.org>