[CPUFREQ] e_powersaver: Allow user to lower maximum voltage

Add new module option "set_max_voltage".
One of the lessons learned from Adaptive Powersaver is that voltage values
returned by processor are for worst case scenario. But required voltage
is changing with CPU temperature. And even processors produced in the same
batch can have different minimum voltage necessary for stable work at
specified frequency.
On Elonex Webbook, once system starts, temperature never drops below
48 deg. C. Loading module after systems start allows user to lower CPU
voltage and still have stable system.
Sadly C7 doesn't allow code to set frequency or voltage from outside limits.
If you ask it to set voltage lower then minimum it will ignore you. Thats
why it isn't possible to change minimum voltage for minimum frequency too.
Changing maximum voltage on Elonex Webbook leads to very good results. Looks
like VIA C7 1.6GHz 1084mV can safetly run at 892mV. This means 83% of
orginal value. If same percentage applies to power generated it means 12.5W
in the place of 15W. Not much, but it is better then nothing.
Only C7-M makes it possible.
If voltage is too low by 16mV or more you will experience kernel panic.
If voltage is too low by 32mV or more you will experience system freeze.

Signed-off-by: Rafał Bilski <rafalbilski@interia.pl>
Signed-off-by: Dave Jones <davej@redhat.com>