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

arch/mips/Makefile

@@ -380,6 +380,7 @@ define archhelp
 	echo '  vmlinux.ecoff        - ECOFF boot image'
 	echo '  vmlinux.bin          - Raw binary boot image'
 	echo '  vmlinux.srec         - SREC boot image'
+	echo '  vmlinux.32           - 64-bit boot image wrapped in 32bits (IP22/IP32)'
 	echo '  vmlinuz              - Compressed boot(zboot) image'
 	echo '  vmlinuz.ecoff        - ECOFF zboot image'
 	echo '  vmlinuz.bin          - Raw binary zboot image'