drm/rockchip: add ability to handle external dphys in mipi-dsi

While the common case is that the dsi controller uses an internal dphy,
accessed through the phy registers inside the dsi controller, there is
also the possibility to use a separate dphy from a different vendor.

One such case is the Rockchip px30 that uses a Innosilicon Mipi dphy,
so add the support for handling such a constellation, including the pll
also getting generated inside that external phy.

changes in v5:
- rebased on top of 5.5-rc1
- merged with dsi timing change to prevent ordering conflicts

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209143130.4553-5-heiko@sntech.de