config: Simplify the definitions of the Voltage and Current params.

config: Simplify the definitions of the Voltage and Current params.

These had a lot of code which duplicated what was already in the
Float param value class. Also, printing into the ini file with "%f"
forces python to truncate values which require more precision than the
fixed float format supports.

Change-Id: Iad9623b71a31d17b69c184082585dcbb881eaa20
Reviewed-on: https://gem5-review.googlesource.com/5622
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>