[ARM] Orion: DRAM mapping granularity is 64KiB, not 16MiB

The DRAM base address and size fields in the CPU's MBUS bridge have
64KiB granularity, instead of the currently used 16MiB.  Since all
of the currently supported MBUS peripherals support 64KiB granularity
as well, this patch changes the Orion address map code to stop
rounding base addresses down and sizes up to multiples of 16MiB.

Found by Ke Wei <kewei@marvell.com>.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Russell King <linux@arm.linux.org.uk>