ARM: mvebu: support running big-endian

Add indication we can run these cores in BE mode, and ensure that the
secondary CPU is set to big-endian mode in the initialisation code as
the initial code runs little-endian.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>