PCI: aardvark: Enable MSI-X support

PCI: aardvark: Enable MSI-X support

According to PCI 3.0 specification, sending both MSI and MSI-X interrupts
is done by DWORD memory write operation to doorbell message address. The
write operation for MSI has zero upper 16 bits and the MSI interrupt number
in the lower 16 bits, while the write operation for MSI-X contains a 32-bit
value from MSI-X table.

Since the driver only uses interrupt numbers from range 0..31, the upper
16 bits of the DWORD memory write operation to doorbell message address
are zero even for MSI-X interrupts. Thus we can enable MSI-X interrupts.

Testing proves that kernel can correctly receive MSI-X interrupts from PCIe
cards which supports both MSI and MSI-X interrupts.

Link: https://lore.kernel.org/r/20220110015018.26359-13-kabel@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>