UPSTREAM: cgroup: add cgroup_parse_float()

cgroup already uses floating point for percent[ile] numbers and there
are several controllers which want to take them as input.  Add a
generic parse helper to handle inputs.

Update the interface convention documentation about the use of
percentage numbers.  While at it, also clarify the default time unit.

Bug: 120440300
Signed-off-by: Tejun Heo <tj@kernel.org>
(cherry picked from commit a5e112e6424adb77d953eac20e6936b952fd6b32)
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Change-Id: Ic1fcf21d7955eb8edd2e8e91517bca6aef41694f
Signed-off-by: Quentin Perret <qperret@google.com>