ARM: MCPM: make internal helpers private to the core code

This concerns the following helpers:

	__mcpm_cpu_going_down()
	__mcpm_cpu_down()
	__mcpm_outbound_enter_critical()
	__mcpm_outbound_leave_critical()
	__mcpm_cluster_state()

They are and should only be used by the core code now.  Therefore their
declarations are removed from mcpm.h and their definitions are made
static, hence the need to move them before their users which accounts
for the bulk of this patch.

This left the mcpm_sync_struct definition at an odd location, therefore
it is moved as well with some comment clarifications.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Dave Martin <Dave.Martin@arm.com>