MIPS: qemu-mips: add '-M mips' switch to the example usage command

MIPS: qemu-mips: add '-M mips' switch to the example usage command

Using the example command from the README file does
not work as expected. qemu shows a text similar to
the one below and it hangs.

    $ qemu-system-mips -L . -nographic
    Could not open option rom 'pxe-pcnet.rom': No such file or directory
    qemu-system-mips: pci_add_option_rom: failed to find romfile "vgabios-cirrus.bin"
    qemu: terminating on signal 15 from pid 19726

This happens because qemu emulates a Malta board by
default if the machine type is not defined explicitely
on the command line.

For a working test, the '-M mips' switch is required:

    $ qemu-system-mips -M mips -L . -nographic
    Could not open option rom 'vgabios.bin': No such file or directory

    U-Boot 2013.01-rc2-00132-g1e8e648-dirty (Jan 08 2013 - 09:06:42)

    Board: Qemu -M mips CPU: 24Kf proc_id=0x19300
    DRAM:  128 MiB
    ## Unknown flash on Bank 1 - Size = 0x00000000 = 0 MB
    Flash: 0 Bytes
    *** Warning - bad CRC, using default environment

    In:    serial
    Out:   serial
    Err:   serial
    Net:   NE2000
    Hit any key to stop autoboot:  0
    qemu-mips #

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Vlad Lungu <vlad.lungu@windriver.com>