feat(cpus): add pabandon support to the Alto cpu

Alto supports powerdown abandon. That support has flown under the radar
so add it in the same way as other pabandon CPUs.

Change-Id: I15f9e8cd77bf5aa23df8e548eb3d35d5c1f4eb2d
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
diff --git a/lib/cpus/aarch64/cortex_alto.S b/lib/cpus/aarch64/cortex_alto.S
index 69a630d..6d0e08b 100644
--- a/lib/cpus/aarch64/cortex_alto.S
+++ b/lib/cpus/aarch64/cortex_alto.S
@@ -41,6 +41,7 @@
 	sysreg_bit_set CORTEX_ALTO_IMP_CPUPWRCTLR_EL1, \
 		CORTEX_ALTO_IMP_CPUPWRCTLR_EL1_CORE_PWRDN_EN_BIT
 	isb
+	signal_pabandon_handled
 	ret
 endfunc cortex_alto_core_pwr_dwn