arm: at91: mpddr: allow multiple DDR controllers

The mpddr.c depends on ATMEL_BASE_MPDDRC for the base address to configure the controller.
This cannot be used when there is more than one controller (i.e. AT91SAM9G45, AT91SAM9M10).

Signed-off-by: Erik van Luijk <evanluijk@interact.nl>
[remove 'new blank line at EOF']
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>