MIPS: lantiq: Convert the fpi bus driver to a platform_driver

MIPS: lantiq: Convert the fpi bus driver to a platform_driver

Instead of hacking the configuration of the FPI bus into the arch code
add an own bus driver for this internal bus. The FPI bus is the main
bus of the SoC. This bus driver makes sure the bus is configured
correctly before the child drivers are getting initialized. This driver
will probably also be used on different SoCs later.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: john@phrozen.org
Cc: p.zabel@pengutronix.de
Cc: kishon@ti.com
Cc: mark.rutland@arm.com
Cc: linux-mips@linux-mips.org
Cc: linux-mtd@lists.infradead.org
Cc: linux-watchdog@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-spi@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17122/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>