ARM: vexpress/TC2: Convert OPP voltage to uV before storing

The SPC stores voltage in mV while the code assumes it was returning
uV. Convert the returned voltage to uV before storing. Also fix the
comment depicting voltage to uV.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>