regulator: rk808: add #include for gpiod functions

This fixes a build problem on mips found by the kbuild test robot:

drivers/regulator/rk808-regulator.c: In function 'rk808_buck1_2_get_voltage_sel_regmap':
drivers/regulator/rk808-regulator.c:97:2: error: implicit declaration of function 'gpiod_get_value' [-Werror=implicit-function-declaration]
  if (IS_ERR(gpio) || gpiod_get_value(gpio) == 0)
  ^

Fixes: bad47ad2 ("regulator: rk808: fixed the overshoot when adjust voltage")
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Mark Brown <broonie@kernel.org>