MIPS: ath79: Add a machine entry for booting OF machines

MIPS: ath79: Add a machine entry for booting OF machines

As I'm using a board with a broken old bootloader I hardcoded the
mips_machtype and did't notice that the machine entry was still
missing.

[ralf@linux-mips.org: Fixed spelling message noticed by Sergei Shtylyov
<sergei.shtylyov@cogentembedded.com>.]

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11503/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>