oxnas: fix syntax in ox820-akitio.dts

This commit introduced a syntax error in ox820-akitio.dts which is
fixed now:
commit 5cde94d9ab577c5ab68fc71e15a05d1bda5041f2
Author: Daniel Golle <daniel@makrotopia.org>
Date:   Sat Sep 24 01:14:53 2016 +0200
    oxnas: backport upstream NAND driver

This caused the folowing error message in the build bot:
Error: arch/arm/boot/dts/ox820-akitio.dts:146.3-147.1 syntax error
FATAL ERROR: Unable to parse input tree
scripts/Makefile.lib:293: recipe for target 'arch/arm/boot/dts/ox820-akitio.dtb' failed
make[5]: *** [arch/arm/boot/dts/ox820-akitio.dtb] Error 1

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>