PCI: mvebu: Setup BAR0 in order to fix MSI

PCI: mvebu: Setup BAR0 in order to fix MSI

According to the Armada XP datasheet, section 10.2.6: "in order for
the device to do a write to the MSI doorbell address, it needs to write
to a register in the internal registers space".

As a result of the requirement above, without this patch, MSI won't
function and therefore some devices won't operate properly without
pci=nomsi.

This requirement was not present at the time of writing this driver
since the vendor u-boot always initializes all PCIe controllers
(incl. BAR0 initialization) and for some time, the vendor u-boot was
the only available bootloader for this driver's SoCs (e.g. A38x,A37x,
etc).

Tested on an Armada 385 board on mainline u-boot (2020.4), without
u-boot PCI initialization and the following PCIe devices:
        - Wilocity Wil6200 rev 2 (wil6210)
        - Qualcomm Atheros QCA6174 (ath10k_pci)

Both failed to get a response from the device after loading the
firmware and seem to operate properly with this patch.

Link: https://lore.kernel.org/r/20200623060334.108444-1-sh@tkos.co.il
Signed-off-by: Shmuel Hazan <sh@tkos.co.il>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>