[ARM] Orion: fix PCIe inbound window programming when RAM size is not a power of two

The PCIe inbound window size is supposed to be a power of two.  If
the total amount of RAM installed in the system is not a power of two,
round it up such that it is.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>