[ARM] Orion: make window setup a little more safe

Currently, Orion window setup uses hardcoded window indexes for each
of the boot/cs0/cs1/cs2/PCIe WA windows.  The static window allocation
used can clash if board support code will ever attempt to configure
both a dev2 and a PCIe WA window, as both of those use CPU mbus window
#7 at present.

This patch keeps track of the last used window, and opens subsequently
requested windows sequentially, starting from 4.  (Windows 0-3 are used
as MEM/IO windows for the PCI/PCIe buses.)

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