A problem when using 2nd stage booting on mvebu is that the first bootloader
already switched the register window location from 0xd0000000 to
0xf1000000 by writing to 0xd0000080. When the second bootloader also
tries to do this switch it writes to the wrong location resulting in an
exception and so a boot failure.

For this reason the base address of the register window is passed in the
barebox header and picked up from there by early code. In a further
patch bootm is taught to put the actual position of the window there for
the second bootloader to finally make second stage booting work.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>