arm64: Add dtb files to archclean rule

As dts files have been reorganised under vendor subdirs, dtb files
cannot be removed with "make distclean" now. Thus, this patch moves
dtb files under archclean rule and removes unnecessary entries.

Cc: Robert Richter <rrichter@cavium.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jungseok Lee <jungseoklee85@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit c7c52e482975cb9c390471df35ab85e86dbc5916)
Signed-off-by: Alex Shi <alex.shi@linaro.org>