mfd: Separate out STMPE controller and interface specific code
Few STMPE controller can have register interface over SPI or I2C. Current
implementation only supports I2C and all code is present in a single file
stmpe.c. It would be better to separate out I2C interface specific code from
controller specific code. Later SPI specific code can be added in a separate
file.
This patch separates out I2C and controller specific code into separate files,
making stmpe.c independent of I2C.