python: Refactor toNum to support a selection of units

python: Refactor toNum to support a selection of units

Add support for matching one of several different units in toNum. The
units parameter can now either be a tuple or a string describing the
supported unit(s). The function now returns a (magnitude, unit) tuple.

Change-Id: I683819722a93ade91a6def2bfa77209c29b4b39e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39217
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>