ARM: vexpress/dcscb: add CPU use counts to the power up/down API implementation

It is possible for a CPU to be told to power up before it managed
to power itself down.  Solve this race with a usage count to deal
with this possibility as mandated by the MCPM API definition.

Signed-off-by: nicolas Pitre <nico@linaro.org>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>