dt-bindings: mfd: hi6421: Add hi6421v530 compatible string

Add compatible string for HiSilicon Hi6421v530 PMIC.
Signed-off-by: NGuodong Xu <guodong.xu@linaro.org>
Acked-by: NLee Jones <lee.jones@linaro.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NWei Xu <xuwei5@hisilicon.com>

Documentation/devicetree/bindings/mfd/hi6421.txt

 * HI6421 Multi-Functional Device (MFD), by HiSilicon Ltd.
 
 Required parent device properties:
-- compatible	: contains "hisilicon,hi6421-pmic";
+- compatible    : One of the following chip-specific strings:
+	"hisilicon,hi6421-pmic";
+	"hisilicon,hi6421v530-pmic";
 - reg		: register range space of hi6421;
 
 Supported Hi6421 sub-devices include: