irqchip/gic: Update supports_deactivate static key to modern api

No changes in semantics -- key init is true; replace

static_key_slow_dec       with   static_branch_disable
static_key_true           with   static_branch_likely

The first is because we never actually do any couterpart incs,
thus there is really no reference counting semantics going on.
Use the more proper static_branch_disable() construct.

Also added a '_key' suffix to supports_deactivate, for better
self documentation.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>