mfd: Add driver for STA2X11 MFD block

This also introduces <asm/sta2x11.h> to export a function that is in
the base sta2x11 support patches. The header will increase with other
prototypes and constants over time.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>