hw/mips/mipssim: Use MMIO serial device on fake ISA I/O

hw/mips/mipssim: Use MMIO serial device on fake ISA I/O

The 'mipssim' is not a real hardware, it is a simulator.

There is an ISA MMIO space mapped at 0x1fd00000, however
this is not a real ISA bus (no ISA IRQ). So can not use
the TYPE_ISA_SERIAL device...
Instead we have been using a plain MMIO device, but named
it IO.

TYPE_SERIAL_IO is a subset of TYPE_SERIAL_MM, using
regshift=0 and endianness=DEVICE_LITTLE_ENDIAN.

Directly use the TYPE_SERIAL_MM device, enforcing the
regshift/endianness values. 'regshift' default is already
'0'. 'endianness' is meaningless for 8-bit accesses.

This change breaks migration back compatibility, but
this is not an issue for the mipssim machine.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200907011538.818996-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>