Update MAINTAINERS file to include myself in AArch64 port.
ChangeLog:
* MAINTAINERS: Add myself to AArch64 pot.
aarch64 ldp/stp Alex Coplan <alex.coplan@arm.com>
aarch64 port Richard Earnshaw <richard.earnshaw@arm.com>
aarch64 port Richard Sandiford <richard.sandiford@arm.com>
+aarch64 port Tamar Christina <tamar.christina@arm.com>
aarch64 port Kyrylo Tkachov <ktkachov@nvidia.com>
alpha port Richard Henderson <rth@gcc.gnu.org>
amdgcn port Julian Brown <julian@codesourcery.com>