regulator: spacemit-p1: Support board power tree

regulator: spacemit-p1: Support board power tree

Merge series from Guodong Xu <guodong@riscstar.com>:

Patch 1, 2 and 3 (previously 2-4) enable flexible power tree
configurations for the SpacemiT P1 PMIC. Hardcoded supply assumptions
are replaced with explicit devicetree properties. PMIC supply connections
are board-design decisions. Moving this to DT allows supporting varied
topologies without driver modifications.

The supply binding change is an ABI change. The breakage is acceptable:
Yixun Lan checked the DTS tree queued for v6.20 and found no consumers
of the P1/PMIC regulator yet [1]. For the two K1 boards in-tree
(BPI-F3 and Jupiter), initial power settings come from boot firmware and
a probe failure of pmic node "spacemit,p1" has minimal impact.

In v4, the old "vin-supply" property is dropped from the binding
document as the updated driver no longer parses it and there is no
fallback logic. Only the per-rail names ("vin1-supply", "vin2-supply",
...) are supported going forward.

Intermittent dtbs_check warnings are expected while the binding and DTS
changes land through different trees, but will resolve once both are
merged.

Link: https://lore.kernel.org/lkml/20260125110333-GYD71302@gentoo.org/