opp: ti-opp-supply: Dynamically update u_volt_min

The voltage range (min, max) provided in the device tree is from
the data manual and is pretty big, catering to a wide range of devices.
On a i2c read/write failure the regulator_set_voltage_triplet function
falls back to set voltage between min and max. The min value from Device
Tree can be lesser than the optimal value and in that case that can lead
to a hang or crash. Hence set the u_volt_min dynamically to the optimal
voltage value.

Cc: 4.16+ <stable@vger.kernel.org> # v4.16+
Fixes: 9a835fa6 ("PM / OPP: Add ti-opp-supply driver")
Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>