qemuarm64: add compatibility with -rt

qemuarm64: add compatibility with -rt

We have an rt BSP definition for preempt-rt and qemuarm, and it boots
with 4.18, so we add it to the compatible machines:

  qemuarm64 login: root
  root@qemuarm64:~# uname -a
  Linux qemuarm64 4.18.3-rt1-yocto-preempt-rt #1 SMP PREEMPT RT Tue Aug 28
  14:10:55 UTC 2018 aarch64 aarch64 aarch64 GNU/Linux

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>