ARM: imx_v6_v7_defconfig: Increase CMA size

Increase the CMA size to 64 MiB, otherwise it isn't possible to use
etnaviv driver on systems with 1920x1080 panel due to insufficient
memory .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>