The regulator is shared between several devices on some platforms,
if cpu target rate is equal to initial rate, the min_uV and max_uV
of cpu regulator will be always equal to zero, other devices may
set a low voltage which can't meet cpu initial rate.
Change-Id: I97e4815a2422417cd9bd605c93138f5ffda9cc86
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>