hikey960: remove mailbox driver
hikey960: remove mailbox driver

Since this mailbox driver is abandoned, remove it.

Signed-off-by: Haojian Zhuang <979139d45d262f0480136b0da512e0afa0fe79d3@linaro.org>

achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware