ARM: dts: rename oxnas dts files

Other platforms' device-tree files start with a platform prefix, such as
sun7i-a20-*.dts or at91-*.dts.
This naming scheme turns out to be handy when using multi-platform build
systems such as OpenWrt.
Prepend oxnas files with their platform prefix to comply with the naming
scheme already used for most other platforms.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>