MIPS: Ci20: Enable SPI/GPIO driver

Enable CONFIG_SPI_GPIO in ci20_defconfig, in order to make use of the
spi-gpio driver in a further commit.

[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/19488/
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