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

Add compatible string for HiSilicon Hi6421v530 PMIC.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>