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>