ARM: dts: lpc32xx: Fix SPI controller node names

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

Cc: Vladimir Zapolskiy <vz@mleia.com>
Cc: Sylvain Lemieux <slemieux.tyco@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>