arm64: defconfig: Reduce default CMA pool from to 8 MB for trusted VM

The trusted VM reserves 32 MB of memory for CMA usage. Given
that the CMA requirement for the trusted VM is much lower than
32 MB, reduce the default CMA pool size to 8 MB for the trusted
VM, to give the kernel more memory for non-movable allocations.

Change-Id: Ib205767979fdf4655a660556c3546e7d5ffda8fb
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>