MAINTAINERS: ARM64 arch: add @JiafeiPan as collaborator

I have already contributed a lot of patches to ARM64 arch regarding
of booting, cache, SMP, GIC etc, and currently I am NXP MPU maintainer,
so I would like to add me to be collaborator of ARM64 arch to help to
review related patches.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>