Often enough the exception vectors are not on TEXT_BASE (for example
on i.MX SoCs in internal boot mode), so the board specific code did
not map the exception vectors to 0x0 but whatever happens to be on
TEXT_BASE. Also, the current section-only mapping requires the
exception vectors to be on a 1MB boundary.
Instead, create the possibility to create second level tables and
use this to map a copy of the exception vectors in a board
independent way.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>