pci: pci_mvebu: Remove unused DECLARE_GLOBAL_DATA_PTR

The global data pointer is not used in this driver, remove it's
declaration.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>