fix(rpi3): tighten platform pwr_domain_pwr_down_wfi behaviour

Platforms which implement pwr_domain_pwr_down_wfi differ substantially
in behaviour. However, different cpus require similar sequences to power
down. This patch tightens the behaviour of these platforms to end on a
wfi loop after performing platform power down. This is required so that
platforms behave more consistently on power down, in cases where the wfi
can fall through.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ie29bd3a5e654780bacb4e07a6d123ac6d2467c1f