dts, arm64: Add dtbs_install make target

This adds the dtbs_install make target to arm64. The target has been
introduced already to arch/arm with the following commit:

 f4d4ffc0 kbuild: dtbs_install: new make target

Implementation for arm64 is the same as for arm.

With 'dtbs_install' all config enabled dtb files are installed to
either the INSTALL_DTBS_PATH directory or the default location:

 $INSTALL_PATH/dtbs/$KERNELRELEASE

Signed-off-by: Robert Richter <rrichter@cavium.com>