dt-bindings: sy8824x: Document SY20278 support

SY20276 is an I2C-controlled adjustable voltage regulator made by
Silergy Corp. The differences between SY8824C and SY20278 are
different regs for mode/enable.
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20190827163754.170cf130@xhacker.debianSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/sy8824x.txt

@@ -5,6 +5,7 @@ Required properties:
 	"silergy,sy8824c"
 	"silergy,sy8824e"
 	"silergy,sy20276"
+	"silergy,sy20278"
 - reg: I2C slave address
 
 Any property defined as part of the core regulator binding, defined in