board/BuR: provide real clock-frequency instead a divider

Actual am335x-fb implementation takes now a real clock frequency instead
a divider. So this component doesn't need to know anymore some base
frequency of the LCDC, we simply provide the pixel-clock frequency.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Anatolij Gustschin <agust@denx.de>