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>