arm: a37xx: pci: Do not allow setting ROM BAR on PCI Bridge

arm: a37xx: pci: Do not allow setting ROM BAR on PCI Bridge

PCI Bridge which represents aardvark PCIe Root Port has Expansion ROM Base
Address register at offset 0x30 but its meaning is different than PCI's
Expansion ROM BAR register. Only address format of register is same.

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

Fixes: 7b758b439474 ("arm: a37xx: pci: Add support for accessing PCI Bridge on root bus")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>