util: Simplify/consolidate the python conversion module.

util: Simplify/consolidate the python conversion module.

The python conversion module was really repetitive and fragmented,
where some types of conversions use common code, and some use hand
written case statements which did something very similar. Also, some
types like Voltage could only handle V and mV but no other scaling
prefix.

This change restructures the module to centralize a lot of the unit
handling code into toFloat, and makes the various other functions use
it.

Change-Id: Ic8529203cc226c9b551b8535a444e3f2f25ad1eb
Reviewed-on: https://gem5-review.googlesource.com/5621
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>