dt-bindings: mfd: hisilicon,hi6421-spmi-pmic: Fix up binding reference

dt-bindings: mfd: hisilicon,hi6421-spmi-pmic: Fix up binding reference

Fix up the SPMI PMIC binding document free text reference which
erroneously referred to itself rather than the parent SPMI controller
binding as intended.

Fixes: 9e5917288545 ("dt: document HiSilicon SPMI controller and mfd/regulator properties")
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231206111754.7410-2-johan+linaro@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>