mfd: Register ab8500 devices using the newly DT:ed MFD API

Now the MFD API is Device Tree aware we can use it for platform
registration again, even when booting with DT enabled. To aid in
Device Node pointer allocation we provide each cell with the
associative compatible string.

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