mfd: Add support for STMPE SPI interface

Few STMPE controller can have register interface over SPI or I2C. Current
implementation only supports I2C.

This patch adds support for SPI interface for accessing STMPE's address space.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>