video, ipu: make ldb clock frequency overwritable through board code

the ldb clock can be setup in board code (for example set through PLL5).
Update the ldb_clock rate also through board code.

This should be removed, if a clock framework is availiable.

Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Eric Nelson <eric.nelson@boundarydevices.com>