ARM: dts: ste: Fix SPI controller node names

[ Upstream commit 2f967f9e9fa076affb711da1a8389b5d33814fc6 ]

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

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