drm/rockchip: dw_hdmi: rename vpll clock to reference clock

"vpll" is a misnomer. A clock input to a device should be named after
the usage in the device, not after the clock that drives it. On the
rk3568 the same clock is driven by the HPLL.
To fix that, this patch renames the vpll clock to ref clock. The clock
name "vpll" is left for compatibility to old device trees.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-5-s.hauer@pengutronix.de