update synopsys driver for platform/devicetree support

[Ben Dooks]

This is a series to allow the Synopsys PWM driver to be built for
device-tree/platform usage.

An open question is whether there should be some form of standard
property for the number of PWMs that each device supports.

Note, the patches 5 and 6 could be grouped together as patch
6 undoes some of the bits in 5 for configuration of clock rate.