qemuarm64-secureboot: changed format of memory to MB

When running runqemu the memory argument in the kernel
commandline arguments expects megabytes, so with the previous
format "-m 1GM" ended up being passed.

Change-Id: I5d3bc9d5ce93fa3da5885c99feb99ce80367c993
Signed-off-by: Brett Warren <brett.warren@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>