drm/panel: simple: add panel-dpi support

The panel-dpi compatible is a fallback that
allows the DT to specify the timing.

When matching panel-dpi expect the device tree to include the
timing information for the display-panel.

Background for this change:
There are a lot of panels and new models hits the market very often.
It is a lost cause trying to chase them all and users of new panels
will often find them in situations that the panel they ues are not
supported by the kernel.
On top of this a lot of panels are customized based on customer
specifications.

Including the panel timing in the device tree allows for a simple
way to describe the actual HW and use this description in a generic
way in the kernel.
This allows uses of proprietary panels, or panels which are not
included in the kernel, to specify the timing in the device tree
together with all the other HW descriptions.
And thus, using the device tree it is then easy to add support
for an otherwise unknown panel.

The current support expect panels that do not require any
delays for prepare/enable/disable/unprepare.

Oleksandr Suvorov replied:
I've just tested this patch on Apalis iMX6Q and Colibri iMX7D using
panel settings from the following patch:
https://lore.kernel.org/linux-arm-kernel/20200115123401.2264293-4-oleksandr.suvorov@toradex.com/

It works for me, thanks!

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Tested-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200216181513.28109-6-sam@ravnborg.org

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