rpmsg: char: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/rpmsg/rpmsg_char.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240604-md-drivers-rpmsg_char-v1-1-675453267fc6@quicinc.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>