regulator: pf8x00: add a doc for the module

pf8x00 module build was not documented.
Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20210114174714.122561-2-adrien.grassein@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/Kconfig

@@ -846,6 +846,10 @@ config REGULATOR_PF8X00
 	  Say y here to support the regulators found on the NXP
 	  PF8100/PF8121A/PF8200 PMIC.
 
+	  Say M here if you want to support for the regulators found
+	  on the NXP PF8100/PF8121A/PF8200 PMIC. The module will be named
+	  "pf8x00-regulator".
+
 config REGULATOR_PFUZE100
 	tristate "Freescale PFUZE100/200/3000/3001 regulator driver"
 	depends on I2C && OF