rpmsg: Move the rpmsg control device from rpmsg_char to rpmsg_ctrl

Create the rpmsg_ctrl.c module and move the code related to the
rpmsg_ctrldev device in this new module.

Add the dependency between rpmsg_char and rpmsg_ctrl in the
kconfig file:

1) RPMSG_CTRL can set as module or built-in if
  RPMSG=y || RPMSG_CHAR=y || RPMSG_CHAR=n

2) RPMSG_CTRL can not be set as built-in if
   RPMSG=m || RPMSG_CHAR=m

Note that RPMGH_CHAR and RPMSG_CTRL can be activated separately.
Therefore, the RPMSG_CTRL configuration must be set for backwards compatibility.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220124102524.295783-4-arnaud.pouliquen@foss.st.com