drm/rockchip: lvds: Add PX30 support

Introduce PX30 LVDS support. This means adding the relevant helper
functions, a specific probe and also the initialization of a specific
PHY.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224143900.23567-10-miquel.raynal@bootlin.com