mfd: Kontron PLD mfd driver

Add core MFD driver for the on-board PLD found on some Kontron embedded
modules. The PLD device may provide functions like watchdog, GPIO, UART
and I2C bus.

The following modules are supported:
	* COMe-bIP#
	* COMe-bPC2 (ETXexpress-PC)
	* COMe-bSC# (ETXexpress-SC T#)
	* COMe-cCT6
	* COMe-cDC2 (microETXexpress-DC)
	* COMe-cPC2 (microETXexpress-PC)
	* COMe-mCT10
	* COMe-mSP1 (nanoETXexpress-SP)
	* COMe-mTT10 (nanoETXexpress-TT)

Signed-off-by: Kevin Strasser <kevin.strasser@linux.intel.com>