mailbox: Hi6220: add mailbox driver

Add driver for Hi6220 mailbox, the mailbox communicates with MCU; for
sending data, it can support two methods for low level implementation:
one is to use interrupt as acknowledge, another is automatic mode which
without any acknowledge. These two methods have been supported in the
driver. For receiving data, it will depend on the interrupt to notify
the channel has incoming message.

Now mailbox driver is used to send message to MCU to control dynamic
voltage and frequency scaling for CPU, GPU and DDR.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>