mfd: Add STMPE I/O Expander support

mfd: Add STMPE I/O Expander support

Add support for the STMPE family of I/O Expanders from
STMicroelectronics.  These devices include upto 24 gpios and a varying
selection of blocks, including PWM, keypad, and touchscreen controllers.
This patch adds the MFD core.

[[email protected]: fix stmpe811 enable hook]
[[email protected]: add touchscreen platform data]
Acked-by: Luotao Fu <[email protected]>
Acked-by: Linus Walleij <[email protected]>
Signed-off-by: Rabin Vincent <[email protected]>
Signed-off-by: Samuel Ortiz <[email protected]>