MIPS: IP22/IP32: Add line to arch/mips/Makefile archhelp about vmlinux.32

Building a 64bit kernel for the SGI O2 (IP32) and the SGI Indy (IP22) uses
the 'vmlinux.32' target, which converts the output 64-bit 'vmlinux' image
into a 32-bit wrapped image.  This is needed for certain revisions of the
IP22 and IP32 ARCS PROMs to boot correctly, but this target is missing
from the 'archhelp' info that is emitted by 'make help'.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Cc: Linux MIPS List <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7991/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>