ARM: dts: OMAP3-N950-N9: Enable SSI module

The Nokia N950 and Nokia N9 have a modem attached to their
first ssi port. This change adds all necessary information
to initialize the SSI module, but does not yet add the
modem information.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>