arm: plat-orion: make bridge_virt_base non-const to support DT use case

For the Armada 370 and XP SoCs where the DT is used, we need to fill
at runtime the bridge_virt_base field on the
orion_addr_map_cfg. Therefore, remove the 'const' qualifier on this
field.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>