media: platform: synopsys: VIDEO_SYNOPSYS_HDMIRX should depend on ARCH_ROCKCHIP

For now, the Synopsys HDMI HDMI RX Controller is only supported on
Rockchip RK3588 SoCs.  Hence add a dependency on ARCH_ROCKCHIP, to
prevent asking the user about this driver when configuring a kernel
without Rockchip SoC support.

Fixes: 7b59b132 ("media: platform: synopsys: Add support for HDMI input driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Shreeya Patel <shreeya.patel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>