ARM: orion5x: fix Makefile for linkstation-lschl.dtb

The rename of orion5x-lschl.dts needs to be reflected in the Makefile:

make[3]: *** No rule to make target 'arch/arm/boot/dts/orion5x-lschl.dtb', needed by '__build'.

Fixes: 6cfd3cd8

 ("ARM: dts: orion5x-lschl: More consistent naming on linkstation series")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>