mfd: Add devices platform data when the cell data size is not 0

When the cell data_size is 0, the resulting platform_data pointer will be
set to ZERO_SIZE_PTR. That could be misleading for device drivers running
a NULL check on thei platform_data pointer before dereferencing it.

Signed-off-by: Brian Harring <ferringb@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>