mfd: simple-mfd-i2c: Add support for registering devices via MFD cells

More devices are cropping up requiring only Regmap initialisation and
child registration functionality. We currently only support that if
all required devices are represented by their own Device Tree nodes
complete with compatible strings.

However, not everyone is happy with adding empty nodes that provide no
additional device information into the Device Tree.

Rather than have a plethora of mostly empty, function-less drivers in
MFD, we'll support those simple cases in here instead via MFD cells.

Cc: Mark Brown <broonie@kernel.org>
Tested-by: Michael Walle <michael@walle.cc>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Tested-by: Alistair Francis <alistair@alistair23.me>
Signed-off-by: Lee Jones <lee.jones@linaro.org>