media: platform: rename am437x/ to ti/am437x/

As the end goal is to have platform drivers split by vendor,
rename am437x/ to ti/am437x/.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>