mfd: ab8500-core: Pass GPADC compatible string to MFD core

When booting with Device Tree enabled the MFD core uses each device's
compatible string to find and allocate its associated of_node pointer,
which in turn is passed to the driver via the platform_device struct.
Without it, the driver won't be able to interrogate the Device Tree or
locate suitable regulators and will most likely fail to probe.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>