dt-bindings: mfd: x-powers,axp152: Document the AXP228 variant

AXP228 is a PMIC used on boards such as the Clockwork ClockworkPi and
DevTerm. Its register map appears to be identical to the AXP221 variant.
The only known difference is in the default values for regulator on/off
states and voltages.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220810013430.27061-1-samuel@sholland.org