mfd: convert PASIC3 to use MFD core

This patch makes htc-pasic3 register the DS1WM and LED cell drivers
through the MFD core infrastructure instead of allocating the platform
devices manually. It also calculates the bus_shift parameter from the
memory resource size.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>