Makefiles: dt: Always set '-@' on ARCH_BCM2835

On the BCM2835 architecture, always add the '-@' option to enable the
generation of symbols, rather than relying on DTC_FLAGS being empty
or correct.

See: https://github.com/raspberrypi/linux/issues/3846



Signed-off-by: Phil Elwell <phil@raspberrypi.com>