pci: pci_mvebu: Replace MBUS_PCI_*_SIZE by resource_size()

Use more appropriate resource_size() function when working with data in
struct resource.

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