ARM: qemu-arm: Bump RAM size in AArch64 MMU table

Now that PCI devices work with highmem-enabled QEMU emulation, bump up
the RAM size in the MMU tables to gain access to the full 255 GB of RAM
potential instead of the puny 3 GB.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Simon Glass <sjg@chromium.org>