hw/mips/gt64xxx_pci: Align the pci0-mem size

One byte is missing, use an aligned size.

    (qemu) info mtree
    memory-region: pci0-mem
      0000000000000000-00000000fffffffe (prio 0, i/o): pci0-mem
                                      ^

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <20190624222844.26584-8-f4bug@amsat.org>