hw/mips: MIPS Boston board support

Introduce support for emulating the MIPS Boston development board. The
Boston board is built around an FPGA & 3 PCIe controllers, one of which
is connected to an Intel EG20T Platform Controller Hub. It is used
during the development & debug of new CPUs and the software intended to
run on them, and is essentially the successor to the older MIPS Malta
board.

This patch does not implement the EG20T, instead connecting an already
supported ICH-9 AHCI controller. Whilst this isn't accurate it's enough
for typical stock Boston software (eg. Linux kernels) to work with hard
disks given that both the ICH-9 & EG20T implement the AHCI
specification.

Boston boards typically boot kernels in the FIT image format, and this
patch will treat kernels provided to QEMU as such. When loading a kernel
directly, the board code will generate minimal firmware much as the
Malta board code does. This firmware will set up the CM, CPC & GIC
register base addresses then set argument registers & jump to the kernel
entry point. Alternatively, bootloader code may be loaded using the bios
argument in which case no firmware will be generated & execution will
proceed from the start of the boot code at the default MIPS boot
exception vector (offset 0x1fc00000 into (c)kseg1).

Currently real Boston boards are always used with FPGA bitfiles that
include a Global Interrupt Controller (GIC), so the interrupt
configuration is only defined for such cases. Therefore the board will
only allow use of CPUs which implement the CPS components, including the
GIC, and will otherwise exit with a message.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>
[yongbok.kim@imgtec.com:
  isolated boston machine support for mips64el.
  updated for recent Chardev changes.
  ignore missing bios/kernel for qtest.
  added default -drive to if=ide explicitly.
  changed default memory size into 1G due to make check failure
  on 32-bit hosts]
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
