arm: dts: spear600: Add ssp controller nodes

The SPEAr600 has three Synchronous serial port to enables synchronous
serial communication with slave or master peripherals (SPI). Lets add these
nodes to be able to use them.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>