ARM: dts: realview: Fix SPI controller node names

SPI controller nodes should be named 'spi' rather than 'ssp'. Fixing the
name enables dtc SPI bus checks.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>