remoteproc/mtk_scp: Use readl_poll_timeout_atomic() for polling

Convert the usage of an open-coded custom tight poll while loop
with the provided readl_poll_timeout_atomic() macro.

This cleanup brings no functional change.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230104115341.320951-2-angelogioacchino.delregno@collabora.com


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