hw/mips: Deprecate the r4k machine

The r4k machine was introduced in 2005 (6af0bf9c) and its last
logical change was in 2005 (9542611a). After that, one can
count 164 maintenance commits (QEMU API changes) with the
exception of 1 fix in 2015 (memory leak, commit 3ad9fd5a).

This machine was introduced as a proof of concept to run a MIPS
CPU. Two years later, the Malta machine was added (commit
5856de80), modeling a real platform.

Note also this machine has no specification except 5 lines in
the header of this file:

 * emulates a simple machine with ISA-like bus.
 * ISA IO space mapped to the 0x14000000 (PHYS) and
 * ISA memory at the 0x10000000 (PHYS, 16Mb in size).
 * All peripherial devices are attached to this "bus" with
 * the standard PC ISA addresses.

The Linux kernel support for this machine has been dropped more
than 10 years ago in commit 302922e5.

It is time to deprecate this obsolete machine. Users are
recommended to use the Malta board, and its hardware is well
documented.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Acked-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20191125104103.28962-1-philmd@redhat.com>