qemu: fix holding pen mailbox sequence

Before this change, plat_secondary_cold_boot_setup reads wake up mailbox
as a byte array but through 64bit accesses on unaligned 64bit addresses.
In the other hand qemu_pwr_domain_on wakes secondary cores by writing
into a 64bit array.

This change forces the 64bit mailbox format as PLAT_QEMU_HOLD_ENTRY_SIZE
explicitly specifies it.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/plat/qemu/aarch64/plat_helpers.S b/plat/qemu/aarch64/plat_helpers.S
index f287e5b..ed55379 100644
--- a/plat/qemu/aarch64/plat_helpers.S
+++ b/plat/qemu/aarch64/plat_helpers.S
@@ -63,6 +63,7 @@
 func plat_secondary_cold_boot_setup
 	/* Calculate address of our hold entry */
 	bl	plat_my_core_pos
+	lsl	x0, x0, #PLAT_QEMU_HOLD_ENTRY_SHIFT
 	mov_imm	x2, PLAT_QEMU_HOLD_BASE
 
 	/* Wait until we have a go */