phy: phy-mtk-dp: Add driver for DP phy

phy: phy-mtk-dp: Add driver for DP phy

This is a new driver that supports the integrated DisplayPort phy for
mediatek SoCs, especially the mt8195. The phy is integrated into the
DisplayPort controller and will be created by the mtk-dp driver. This
driver expects a struct regmap to be able to work on the same registers
as the DisplayPort controller. It sets the device data to be the struct
phy so that the DisplayPort controller can easily work with it.

The driver does not have any devicetree bindings because the datasheet
does not list the controller and the phy as distinct units.

The interaction with the controller can be covered by the configure
callback of the phy framework and its displayport parameters.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>
[Bo-Chen: Modify reviewers' comments.]
Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220624062725.4095-1-rex-bc.chen@mediatek.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>