drm/exynos: use identifier instead of register offsets

Different revisions of the MIPI-DSI PHY have slightly different register
layouts. Currently, the register layout was stored per platform, which
makes it necessary to define the layout for each new platform.

Keep the register layout in the driver and use identifiers to specify
which register layout shall be used on a platform.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>