MIPS: jz4780: DTS: Probe the spi-gpio driver from devicetree

Make use of the spi-gpio driver to provide SPI support on the Ingenic
JZ4780 SoC using the pins that can be used with the SSI0 device as
GPIOs, until such time as we have support for the Ingenic SPI/SSI
controller.

[paul.burton@mips.com: Rewrite commit message.]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19489/
Cc: James Hogan <jhogan@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org