hikey: psci: use mailbox to emulate low power mode
hikey: psci: use mailbox to emulate low power mode

Use psci's mailbox to emulate low power mode.

Signed-off-by: Leo Yan <a38ad1fbd0b3373e0c012f4d28e16dbce652d487@linaro.org>

96boards/arm-trusted-firmware,96boards/arm-trusted-firmware,vchong/arm-trusted-firmware,vchong/arm-trusted-firmware