MIPS: generic: Fix ranchu_of_match[] termination

ranchu_of_match[] has no terminating element to end the search for a
matching compatible string when the first and only element does not
match, so add one now.

Fixes: f2d0b0d5

 ("MIPS: ranchu: Add Ranchu as a new generic-based board")
Signed-off-by: James Hogan <jhogan@kernel.org>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Miodrag Dinic <miodrag.dinic@mips.com>
Cc: Goran Ferenc <goran.ferenc@mips.com>
Cc: Aleksandar Markovic <aleksandar.markovic@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18581/