siemens: board: clean up products folders vs common

The common folder was initialially created for the common parts of the
products based on draco-am355x board family. These are the product lines
'pxm2', 'rut' and the base line named 'draco'!

Adding the new capricorn-imx8 board family, common was enhanced without
cleanup.
- rename 'common/board.c' to 'common/board_am335x.c'
- add 'common/board_am335x.h' for export to the product lines

Reviewed-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Signed-off-by: Enrico Leto <enrico.leto@siemens.com>