drm/mediatek: Add MT8195 Embedded DisplayPort driver

This patch adds a embedded displayport driver for the MediaTek mt8195 SoC.

It supports the MT8195, the embedded DisplayPort units. It offers
DisplayPort 1.4 with up to 4 lanes.

The driver creates a child device for the phy. The child device will
never exist without the parent being active. As they are sharing a
register range, the parent passes a regmap pointer to the child so that
both can work with the same register range. The phy driver sets device
data that is read by the parent to get the phy device that can be used
to control the phy properties.

This driver is based on an initial version by
Jitao shi <jitao.shi@mediatek.com>

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>
Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220901044149.16782-4-rex-bc.chen@mediatek.com