MIPS: ranchu: Add Ranchu as a new generic-based board

Provide amendments to the MIPS generic platform framework so that
the new generic-based board Ranchu can be chosen to be built.

The Ranchu board is intended to be used by Android emulator. The name
"Ranchu" originates from Android development community. "Goldfish" and
"Ranchu" are terms used for two generations of virtual boards used by
Android emulator. The name "Ranchu" is a newer one among the two, and
this patch deals with Ranchu. However, for historical reasons, some
devices/drivers still contain the name "Goldfish".

MIPS Ranchu machine includes a number of Goldfish devices. The support
for Virtio devices is also included. Ranchu board supports up to 16
Virtio devices which can be attached using Virtio MMIO Bus. This is
summarized in the following picture:

       ABUS
        ||----MIPS CPU
        ||       |                    IRQs
        ||----Goldfish PIC------------(32)--------
        ||                     | | | | | | | | |
        ||----Goldfish TTY------ | | | | | | | |
        ||                       | | | | | | | |
        ||----Goldfish RTC-------- | | | | | | |
        ||                         | | | | | | |
        ||----Goldfish FB----------- | | | | | |
        ||                           | | | | | |
        ||----Goldfish Events--------- | | | | |
        ||                             | | | | |
        ||----Goldfish Audio------------ | | | |
        ||                               | | | |
        ||----Goldfish Battery------------ | | |
        ||                                 | | |
        ||----Android PIPE------------------ | |
        ||                                   | |
        ||----Virtio MMIO Bus                | |
        ||    |    |    |                    | |
        ||    |    |   (virtio-block)--------- |
        ||   (16)  |                           |
        ||    |   (virtio-net)------------------

Device Tree is created on the QEMU side based on the information about
devices IO map and IRQ numbers. Kernel will load this DTB using UHI
boot protocol DTB handover mode.

Signed-off-by: Miodrag Dinic <miodrag.dinic@mips.com>
Signed-off-by: Goran Ferenc <goran.ferenc@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18138/
Signed-off-by: James Hogan <jhogan@kernel.org>