drivers/bus: arm-cci: function to enable CCI ports from early boot code

This provides cci_enable_port_for_self().  This is the counterpart to
cci_disable_port_by_cpu(self).

This is meant to be called from the MCPM machine specific power_up_setup
callback code when the appropriate affinity level needs to be initialized.
The code therefore has to be position independent as the MMU is still off
and it cannot rely on any stack space.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Dave Martin <dave.martin@linaro.org>