remoteproc: xlnx: Fix kernel-doc warnings

Fix kernel-doc warnings generated by following command:

`scripts/kernel-doc -Werror -Wshort-desc -Wall \
drivers/remoteproc/xlnx_r5_remoteproc.c > /dev/null`

warning: missing initial short description on line:
 * struct mbox_info

...

Total 8 warnings fixed

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Link: https://lore.kernel.org/r/20250716213048.2316424-3-tanmay.shah@amd.com


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