ARM: dts: orion5x: fix the missing mtd flash on linkstation lswtgl

MTD flash stores u-boot and u-boot environment on linkstation lswtgl.
The latter one can be easily read/write by u-boot-tools package in Debian.

Fixes: dc57844a

 ("ARM: dts: orion5x: add buffalo linkstation ls-wtgl")
Signed-off-by: Roger Shimizu <rogershimizu@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>