hikey9xx: Fixed incorrect use of kfree to free sreg

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6GS86


CVE: NA

--------------------------------

When memory allocated by using the devm_kmalloc(). the matching
release function should use the devm_kfree() function instead of
the kfree() function.

@sreg allocated:
hi6421_spmi_regulator_probe
  hi6421_spmi_regulator_probe_ldo

@sreg freed:
hi6421_spmi_regulator_remove

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>