arm/arm64: mediatek: Fix mmsys device probing

[matthias.bgg at kernel.org]

Changes since v3:
- use platform device to probe clock driver
- add Acked-by CK Hu for the probe deferred patch

Changes since v2:
- fix kconfig typo (shame on me)
- delete __initconst from mm_clocks as converted to a platform driver
  
Changes since v1:
- add binding documentation
- ddp: use regmap_update_bits
- ddp: ignore EPROBE_DEFER on clock probing
- mfd: delete mmsys_private
- add Reviewed-by and Acked-by tags
 
MMSYS in Mediatek SoCs has some registers to control clock gates (which is 
used in the clk driver) and some registers to set the routing and enable
the differnet blocks of the display subsystem.

Up to now both drivers, clock and drm are probed with the same device tree
compatible. But only the first driver get probed, which in effect breaks
graphics on mt8173 and mt2701.

This patch uses a platform device registration in the DRM driver, which
will trigger the probe of the corresponding clock driver. It was tested on
the bananapi-r2 and the Acer R13 Chromebook.