video, ipu: make ldb_clock configurable

make the ldb_clock configurable through the new define
CONFIG_SYS_LDB_CLOCK. This is needed as the ldb clock is not
always 650000000, for example on the aristainetos2 board,
where the ldb clock derives from PLL5 clock.

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