remoteproc: mtk_scp: Constify buffer passed to scp_send_ipi()

remoteproc: mtk_scp: Constify buffer passed to scp_send_ipi()

scp_send_ipi() should only send the passed buffer, without modifying its
contents, so mark pointer 'buf' as pointer to const.

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-v3-2-4d7fd27f037f@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>