dpll: Prevent duplicate registrations

In the Linux kernel, the following vulnerability has been resolved:

dpll: Prevent duplicate registrations

Modify the internal registration helpers dpll_xa_ref_{dpll,pin}_add() to reject duplicate registration attempts.

Previously, if a caller attempted to register the same pin multiple times (with the same ops, priv, and cookie) on the same device, the core silently increments the reference count and return success. This behavior is incorrect because if the caller makes these duplicate registrations then for the first one dpll_pin_registration is allocated and for others the associated dpll_pin_ref.refcount is incremented. During the first unregistration the associated dpll_pin_registration is freed and for others WARN is fired.

Fix this by updating the logic to return -EEXIST if a matching registration is found to enforce a strict "register once" policy.