ARM: dts: exynos: Align SPI GPIO node name with dtschema in Universal C210

The device tree schema expects SPI controller to be named "spi",
otherwise dtbs_check complain with a warning like:

  arch/arm/boot/dts/exynos4210-universal_c210.dt.yaml: spi-lcd:
    $nodename:0: 'spi-lcd' does not match '^spi(@.*|-[0-9a-f])*$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200830135200.24304-13-krzk@kernel.org