staging: hikey9xx: Kconfig: add regulator dependency

The regulator driver needs it, as otherwise it will produce
errors when creating vmlinux.
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a287238bd0141aa68aae7d4cec4c1174a43b3e53.1597766680.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/hikey9xx/Kconfig

@@ -31,6 +31,7 @@ config MFD_HI6421_SPMI
 config REGULATOR_HI6421V600
 	tristate "HiSilicon Hi6421v600 PMIC voltage regulator support"
 	depends on MFD_HI6421_SPMI && OF
+	depends on REGULATOR
 	help
 	  This driver provides support for the voltage regulators on
 	  HiSilicon Hi6421v600 PMU / Codec IC.