arm: a37xx: pci: Find PCIe controller node by compatible instead of path

Find PCIe DT node by compatible string instead of retrieving it by using
hardcoded DT path.

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