PCI: mvebu: Remove unnecessary use of 'conf_lock' spinlock

Serialization of configuration accesses is provided by 'pci_lock' in
drivers/pci/access.c thus making the driver's 'conf_lock' superfluous.

Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>