arm: a37xx: pci: Do not allow setting bars on PCI Bridge

PCI Bridge which represents Aardvark PCIe Root Port does not have
configurable bars.

So ensure that write operation to bars registers on PCI Bridge is noop and
bars registers always contain zero address which indicates that bars are
unsupported.

After this change U-Boot 'pci bar 0.0.0' command does not show any
allocated bars for PCI Bridge device.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: cb056005 ("arm: a37xx: pci: Add support for accessing PCI Bridge on root bus")
Reviewed-by: Stefan Roese <sr@denx.de>