MLK-12556 dts: i.mx6ul: configure the CMA region by default

MLK-12556 dts: i.mx6ul: configure the CMA region by default

CMA region is a must to avoid the multile memory mapping
for the DMAed memory and also benifit the large continious
phisical memory allocation.

The default value is depend on the target system design and
user cases definition. This is not suitable to put this into
the soc.dtsi, thus we put it into the board DTS.

customer can override the value by changing cma size in DTS file.

Again, customer need set the CMA size correctly according to the
target system. The incorrectly CMA size can cause Linux kernel fail
to boot up.CMA disabled or CMA size set to zero is also not allowed.

Signed-off-by: Jason Liu <jason.hui.liu@nxp.com>