regulator: mcp16502: document lpm as optional

Document LPM pin as optional.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1610028927-9842-2-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mcp16502-regulator.txt

@@ -4,7 +4,8 @@ Required properties:
 - compatible: "microchip,mcp16502"
 - reg: I2C slave address
 - lpm-gpios: GPIO for LPM pin. Note that this GPIO *must* remain high during
-	     suspend-to-ram, keeping the PMIC into HIBERNATE mode.
+	     suspend-to-ram, keeping the PMIC into HIBERNATE mode; this
+	     property is optional;
 - regulators: A node that houses a sub-node for each regulator within
               the device. Each sub-node is identified using the node's
               name. The content of each sub-node is defined by the