dt-bindings: devfreq: Add DT bindings to add a stall cycle count event

This patch adds the DT bindings to specify the event ID needed
to count the stall cycles.

Change-Id: I96fe9582d4aa9ae9a393f0ec7a337eeb6196bb6b
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>