mfd: Remove obsolete ti-ssp driver

The tnetv107x platform is getting removed, so this driver
is not needed any more.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>