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
diff --git a/plat/rpi/common/rpi3_pm.c b/plat/rpi/common/rpi3_pm.c
index 2f86279..d98ac66 100644
--- a/plat/rpi/common/rpi3_pm.c
+++ b/plat/rpi/common/rpi3_pm.c
@@ -187,8 +187,9 @@
 
 	write_rmr_el3(RMR_EL3_RR_BIT | RMR_EL3_AA64_BIT);
 
-	while (1)
-		;
+	while (1) {
+		wfi();
+	}
 }
 
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