power: sequencing: Add pwrseq_is_controllable() API

On some boards a power sequencing target has no host-controllable enable
for its function, for instance when the enable line is not wired up to a
GPIO and is hardwired to an always-on level. The pcie-m2 "uart" target is
one such example: when the M.2 connector does not route the W_DISABLE2#
signal to a host GPIO, its enable/disable are no-ops and the consumer
cannot gate the Bluetooth function at all or exclusively.

Add a generic pwrseq_is_controllable() helper. It reports whether the
target's final unit provides a host-controllable dedicated power actuator.
The target can implement a new optional is_controllable() callback,
reporting whether that actuator is effective on this target (for example
depending on GPIO presence). If the target does not provide the callback,
it is assumed to be controllable.

Note this only describes the target's own enable actuator. It does not
imply that a power-off reaches an electrical OFF state, since a target may
have multiple consumers. It also does not mean that power is uncontrolled
for the target's dependencies: those may still be gated on their own. And
it does not restrict consumers from calling pwrseq_power_off() either,
which remains valid to drop this consumer's vote on the (possibly shared)
resources and dependencies of the target.

Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Link: https://patch.msgid.link/20260904-monza-wireless-v6-3-d8c5042b3efd@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

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