mfd: Ensure AB8500 platform data is passed through db8500-prcmu to MFD Core

When booting via platform code the AB8500 platform data is now passed
in though the DB8500. However, if pdata_size is not set it will not be
subsequently passed onto subordinate devices. This patch correctly
populates pdata_size.

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