[RFC] PCI: rcar: Add bus notifier so we can limit the DMA range

The PCIe DMA controller on RCar Gen2 and earlier is on 32bit bus,
so limit the DMA range to 32bit.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Phil Edworthy <phil.edworthy@renesas.com>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: linux-renesas-soc@vger.kernel.org
To: linux-pci@vger.kernel.org
---
NOTE: I'm aware of https://patchwork.kernel.org/patch/9495895/ , but the
      discussion seems to have gone way off, so I'm sending this as a
      RFC. Any feedback on how to do this limiting properly would be nice.