arm: a37xx: pci: Implement workaround for the readback value of VEND_ID

Marvell Armada 3720 Functional Errata, Guidelines, and Restrictions
document describes in erratum 4.1 PCIe value of vendor ID (Ref #: 243):

    The readback value of VEND_ID (RD0070000h [15:0]) is 1B4Bh, while it
    should read 11ABh.

    The firmware can write the correct value, 11ABh, through VEND_ID
    (RD0076044h [15:0]).

Implement this workaround in U-Boot PCIe controller driver aardvark for
both PCI vendor id and PCI subsystem vendor id.

This change affects PCI vendor id of PCIe root bridge emulated by Linux
kernel. With this change Linux kernel reports correct vendor id 11AB.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>