MIPS: BMIPS: Add support SPI device nodes

MIPS: BMIPS: Add support SPI device nodes

Adds SPI device nodes to BCM7xxx MIPS based SoCs.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14990/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>