power: regulator: scmi: Move regulator subnode hack to scmi_regulator

power: regulator: scmi: Move regulator subnode hack to scmi_regulator

The current code attempts to bind scmi_voltage_domain to regulator subnode
of the SCMI protocol node, so scmi_voltage_domain can then bind regulators
directly to subnodes of its node. This kind of behavior should not be in
core code, move it into scmi_voltage_domain driver code. Let the driver
descend into regulator node and bind regulators to its subnodes.

Fixes: 1f213ee4dbf2 ("firmware: scmi: voltage regulator")
Signed-off-by: Marek Vasut <marex@denx.de>
[Alice Guo: Fix scmi_regulator_bind]
Signed-off-by: Peng Fan <peng.fan@nxp.com>