ARM: zynq: Leave CMA size at default

Setting the default size for the CMA to high breaks platform that don't
have enough memory to satisfy the CMA demand. Leaving it at default
should enable more platforms to get a working base line using the
defconfig.

Reported-by: Stephen MacMahon <stephenm@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>