regulator: s5m8767: Modify parsing method of the voltage table of buck2/3/4

The s5m8767 regulator driver parse always the voltage table of buck2/3/4.
If gpio_dvs feature isn't used and dts haven't included the voltage table
of buck2/3/4, s5m8767 regulator driver return error and file probe state.

This patch check only voltage table of buck on s5m8767_pmic_dt_parse_pdata()
if buck[2-4]_gpiodvs is true.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>