Makefile: Add missing OF_UPSTREAM Makefile for 32bit ARM

Copy dts/upstream/src/arm64/Makefile into dts/upstream/src/arm/Makefile
and create a commit. This makes 32bit ARM buildable with OF_UPSTREAM .

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Adam Ford <aford173@gmail.com> #am3517-evm
Tested-by: Tony Dinh <mibodhi@gmail.com>