arm64: dts: broadcom: Use the .dtb name in the rule, rather than .dts

Commit a7eb2639 ("arm64: dts: broadcom: Add reference to Compute
Module IO Board V3") adds the bcm2837-rpi-cm3-io3.dts file as a target
in the Makefile, rather than the .dtb name. This will skip the
generation of the .dtb file at compile time and will fail the dtbs_install
target.

Fixes: a7eb2639 ("arm64: dts: broadcom: Add reference to Compute Module IO Board V3")
Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Olof Johansson <olof@lixom.net>