ASoC: rockchip: rockchip_sai: #include <linux/platform_device.h> explicitly

ASoC: rockchip: rockchip_sai: #include <linux/platform_device.h> explicitly

Currently that header is only included via:

	<sound/dmaengine_pcm.h> ->
	<sound/soc.h> ->
	<linux/platform_device.h>

which doesn't look reliable, still more in the presence of the comment:

	/* For the current users of sound/soc.h to avoid build issues */

in <sound/soc.h>.

Signed-off-by: Uwe Kleine-König (The Capable Hub) <u.kleine-koenig@baylibre.com>
Reviewed-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Link: https://patch.msgid.link/20260624083708.254517-2-u.kleine-koenig@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>