ARM: mvebu: mark mvebu_hwcc_pci_nb as __maybe_unused

The coherency notifier block is only used when CONFIG_PCI
is enabled, otherwise we get a warning:

arch/arm/mach-mvebu/coherency.c:110:30: warning: 'mvebu_hwcc_pci_nb' defined but not used [-Wunused-variable]

There is no nice way to use an if(IS_ENABLED()) check here to
let the compiler know that it might be used, so let's mark
the structure as __maybe_unused.

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