Orion: initialise mbus DRAM target info on boot

Initialise orion_mbus_dram_info on boot, and prepare for passing
this info into peripheral drivers.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Nicolas Pitre <nico@marvell.com>