ARM: vexpress/dcscb: handle platform coherency exit/setup and CCI

Add the required code to properly handle race free platform coherency exit
to the DCSCB power down method.

The power_up_setup callback is used to enable the CCI interface for
the cluster being brought up.  This must be done in assembly before
the kernel environment is entered.

Thanks to Achin Gupta and Nicolas Pitre for their help and
contributions.

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