mfd: simple-mfd-i2c: Add sl28cpld support

Add the core support for the board management controller found on the
SMARC-sAL28 board.

Also add a virtual symbol which pulls in the simple-mfd-i2c driver and
provide a common symbol on which the subdevice drivers can depend on.

At the moment, this controller is used on the Kontron SMARC-sAL28 board.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Lee Jones <lee.jones@linaro.org>