dt-bindings: sy8824x: Document SY20276 support

SY20276 is an I2C-controlled adjustable voltage regulator made by
Silergy Corp. The differences between SY8824C and SY20276 are
different vsel_min, vsel_step, vsel_count and regs for mode/enable.
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20190827163650.47ed1213@xhacker.debianSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/sy8824x.txt

-SY8824C/SY8824E Voltage regulator
+SY8824C/SY8824E/SY20276 Voltage regulator
 
 Required properties:
 - compatible: Must be one of the following.
 	"silergy,sy8824c"
 	"silergy,sy8824e"
+	"silergy,sy20276"
 - reg: I2C slave address
 
 Any property defined as part of the core regulator binding, defined in