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>
diff --git a/drivers/mfd/stmpe.h b/drivers/mfd/stmpe.h
index 18d89a6..a73f4c1 100644
--- a/drivers/mfd/stmpe.h
+++ b/drivers/mfd/stmpe.h
@@ -120,6 +120,7 @@
 
 #define STMPE811_REG_CHIP_ID		0x00
 #define STMPE811_REG_SYS_CTRL2		0x04
+#define STMPE811_REG_SPI_CFG		0x08
 #define STMPE811_REG_INT_CTRL		0x09
 #define STMPE811_REG_INT_EN		0x0A
 #define STMPE811_REG_INT_STA		0x0B