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The U-Boot port for ST-Ericsson Ux500 is currently only used on the
"stemmy" board, where U-Boot runs after firmware that already sets up
a boot splash screen. This means that the display is already on
and we can just continue using it for U-Boot.

Add a simple driver that simplifies this by reading the display
configuration (e.g. screen size, bpp) from the hardware registers.

It also checks the configured "source synchronization" - for some
displays (usually DSI command mode displays) we need to explicitly
trigger a software sync. This is done through the video_sync()
callback that triggers the sync and wait for completion.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>