arm: a37xx: pci: Fix DT compatible string to Linux' DT compatible

Change DT compatible string for A3700 PCIe from 'marvell,armada-37xx-pcie'
to 'marvell,armada-3700-pcie' to make U-Boot A3700 PCIe DT node compatible
with Linux' DT node.

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