board/BuR: split bur_am335x_common.h into am335x-specific and BuR common parts

board/BuR: split bur_am335x_common.h into am335x-specific and BuR common parts

bur_am335x_common.h today holds all common configuration which is shared
over all B&R boards.

In future we want to bring up boards which are not based on AM335x only
but we still want to have common configuration over all B&R boards
independent from their architecture.

To prepare this we introduce a new file "bur_cfg_common.h", where we
move all common things, which are not architecture specific, from
bur_am335x_common.h.

On B&R am335x boards we include from now:

#include <configs/bur_cfg_common.h>
#include <configs/bur_am335x_common.h>

On other B&R boards, we include only
#include <configs/bur_cfg_common.h>

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Tom Rini <trini@konsulko.com>