dra7xx: Refactor Makefile and filenames to ease addition of new platforms

Factor out TI common Makefile components, no functional changes made, except
removing -f from the common Makefile for directory removal and adding -p
to directory creation.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>