ARM: bL_switcher: synchronize the outbound with the inbound

Let's wait for the inbound CPU to come up and snoop some of the outbound
CPU cache before bringing the outbound CPU down.  That should be more
efficient than going down right away.

Possible improvements might involve some monitoring of the CCI event
counters.

Signed-off-by: Nicolas Pitre <nico@linaro.org>