rpi: Fix device tree path on ARM64

The directory structure of device tree files produced by the kernel's
'make dtbs_install' is different on ARM64, the RPi3 device tree file is
in a 'broadcom' subdirectory there.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>