PCI: rcar: Don't allocate extra memory for the MSI capture address

A long cargo-culted behaviour of PCI drivers is to allocate memory
to obtain an address that is fed to the controller as the MSI
capture address (i.e. the MSI doorbell).

But there is no actual requirement for this address to be RAM.
All it needs to be is a suitable aligned address that will
*not* be DMA'd to.

Since the rcar platform already has a requirement that this
address should be in the first 4GB of the physical address space,
use the controller's own base address as the capture address.

Link: https://lore.kernel.org/r/20210330151145.997953-3-maz@kernel.org


Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>