MIPS: BCM63xx: Add stub to register the SPI platform driver

This patch adds the necessary stub to register the SPI platform driver.
Since the registers are shuffled between the 4 BCM63xx CPUs supported by
this SPI driver we also need to generate the internal register layout and
export this layout for the driver to use it properly.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: grant.likely@secretlab.ca
Cc: spi-devel-general@lists.sourceforge.net
Patchwork: https://patchwork.linux-mips.org/patch/3321/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>