pci: pci_mvebu: Do not allow setting ROM BAR on PCI Bridge

The PCI Bridge which represents mvebu PCIe Root Port has Expansion ROM
Base Address register at offset 0x30 but its meaning is different that
of PCI's Expansion ROM BAR register, although the address format of
the register is the same.

In reality, this device does not have any configurable PCI BARs. So
ensure that write operation into BARs (including Expansion ROM BAR) is a
noop and registers always contain zero address which indicates that BARs
are unsupported.

Fixes: a7b61ab5 ("pci: pci_mvebu: Properly configure and use PCI Bridge (PCIe Root Port)")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>