regulator: bd71815: add select to fix build

Mend the Kconfig for REGULATOR_BD71815 to prevent build errors:

riscv32-linux-ld: drivers/regulator/bd71815-regulator.o: in function `.L0 ':
regulator.c:289: undefined reference to `rohm_regulator_set_dvs_levels'
riscv32-linux-ld: drivers/regulator/bd71815-regulator.c:370: undefined reference to `rohm_regulator_set_dvs_levels'

Fixes: 1aad3900 ("regulator: Support ROHM BD71815 regulators")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/20210523001427.13500-1-rdunlap@infradead.org


Signed-off-by: Mark Brown <broonie@kernel.org>