drm/panthor: Implement L2 power on/off via PWR_CONTROL

drm/panthor: Implement L2 power on/off via PWR_CONTROL

This patch adds common helpers to issue power commands, poll
transitions, and validate domain state, then wires them into the L2
on/off paths.

The L2 power-on sequence now delegates control of the SHADER and TILER
domains to the MCU when allowed, while the L2 itself is never delegated.
On power-off, dependent domains beneath the L2 are checked, and if
necessary, retracted and powered down to maintain proper domain
ordering.

Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Link: https://patch.msgid.link/20251125125548.3282320-5-karunika.choo@arm.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>