Generate more central values in PSUEDO_RANDOM

Generate more values near the mid-point of the range (usually 0.0) when
generating pseudo-random values. If using a simple uniform distribution
most of the times we generate very large or very small numbers, and
almost never numbers in the -1000 to +1000 range.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I09fdb72f6f339d5a03f3bba1c54924fb4073916a