regulator: Fix SY7636A breakage

The MFD for SY7636A still isn't merged so we need a dependency even if
it ends up not having a real driver due to the header file.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/Kconfig

@@ -1199,6 +1199,7 @@ config REGULATOR_STW481X_VMMC
 
 config REGULATOR_SY7636A
 	tristate "Silergy SY7636A voltage regulator"
+	depends on MFD_SY7636A
 	help
 	  This driver supports Silergy SY3686A voltage regulator.