ipu_common: Let the MX6 IPU clock be calculated in run-time

MX6Q/QP IPU operates at 264MHz and MX6DL IPU at 198MHz.

When running a SPL target, which supports multiple MX6 variants we cannot
properly setup the IPU clock frequency via CONFIG_IPUV3_CLK option as
such decision is done in build-time currently.

Remove the CONFIG_IPUV3_CLK option and let the IPU clock frequency be
configured in run-time on mx6.

Reported-by: Eric Nelson <eric@nelint.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
[agust: fixed #endif in cgtqmx6eval.h]
Signed-off-by: Anatolij Gustschin <agust@denx.de>