Orion: only map peripheral register space once

Save some TLB entries by making ioremap() return pointers into the
boot-time Orion peripheral iotable mapping whenever someone tries
to ioremap any part of the Orion peripheral register space.

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