nios2: Add order-only DTC dependency to %.dtb target

The dtc dependency is missing, thus dtc is not built before it's invoked,
resulting in the following problem:

linux-2.6$ make 10m50_defconfig && make 10m50_devboard.dtb
[...]
scripts/kconfig/conf  --silentoldconfig Kconfig
  DTC     arch/nios2/boot/10m50_devboard.dtb
/bin/sh: 1: ./scripts/dtc/dtc: not found
arch/nios2/boot/Makefile:52: recipe for target 'arch/nios2/boot/10m50_devboard.dtb' failed
make[1]: *** [arch/nios2/boot/10m50_devboard.dtb] Error 127
arch/nios2/Makefile:57: recipe for target '10m50_devboard.dtb' failed
make: *** [10m50_devboard.dtb] Error 2

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Romain Perier <romain.perier@free-electrons.com>
Acked-by: Ley Foon Tan <lftan@altera.com>