bus: mvebu-mbus: make mvebu_mbus_syscore_ops static

Make mvebu_mbus_syscore_ops static as it does not get
used or declared outside mvebu-mbus.c to fix the following
sparse warning:

linux/drivers/bus/mvebu-mbus.c:1092:20: warning: symbol 'mvebu_mbus_syscore_ops' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>