mfd: add driver for sequencer serial port

TI's sequencer serial port (TI-SSP) is a jack-of-all-trades type of serial port
device.  It has a built-in programmable execution engine that can be programmed
to operate as almost any serial bus (I2C, SPI, EasyScale, and others).

This patch adds a driver for this controller device.  The driver does not
expose a user-land interface.  Protocol drivers built on top of this layer are
expected to remain in-kernel.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>