regulator: s5m8767: Implement voltage setting for BUCK7/8 regulators

Voltage setting for BUCK7 and BUCK8 was not supported by s5m8767 driver.
If they were configured in DTS then the driver probing would fail with
EINVAL:
[    0.288474] VCC_SUB_1.35V: failed to apply 1350000uV constraint
[    0.288671] s5m8767-pmic s5m8767-pmic: regulator init failed for 35
[    0.294931] s5m8767-pmic: probe of s5m8767-pmic failed with error -22

This patch adds support for BUCK7 and BUCK8 regulators using standard
regmap get/set voltage sel functions.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>