rpmsg: Constify buffer passed to send API

rpmsg: Constify buffer passed to send API

The rpmsg_send(), rpmsg_sendto() and other variants of sending
interfaces should only send the passed data, without modifying its
contents, so mark pointer 'data' as pointer to const.  All users of this
interface already follow this approach, so only the function
declarations have to be updated.

Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260317-rpmsg-send-const-v3-3-4d7fd27f037f@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>