pci: pci_mvebu: Cleanup macro names

pci: pci_mvebu: Cleanup macro names

Use "MVPCIE_" prefix instead of generic "PCIE_" prefix for pci_mvebu.c
specific macros. Define offset macros for Root Port registers and use
standard register macros from pci.h when accessing Root Port registers.

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