arm: dts: Makefile: clean *dtb_HS

TI HS platforms generate *dtb_HS binary blobs and there is no
rule for cleanup. Added entry for cleanup in clean-files target.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>