ARM: vexpress: add shim layer for psci backend on TC2

ARM: vexpress: add shim layer for psci backend on TC2

This patch introduces a shim layer for the TC2 platform which converts
'bL_platform_power_ops' routines to their psci counterparts. The psci
counterparts are implemented by the secure firmware. The shim layer
is used only when Linux is running in non-secure world and the secure
firmware implements psci.

It also introduces the use of a reference count to allow a power up call
to go ahead of a power down call.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>