mailbox: rockchip: Add Rockchip mailbox driver

This driver is found on RK3368 SoCs.

The Mailbox module is a simple APB peripheral that allows both
the Cortex-A53 MCU system to communicate by writing operation to
generate interrupt.
The registers are accessible by both CPU via APB interface.

The Mailbox has the following main features:

1) Support dual-core system: Cortex-A53 and MCU.
2) Support APB interface.
3) Support four mailbox elements, each element includes one data word,
   one command word register and one flag bit that can represent
   one interrupt.
4) Four interrupts to Cortex-A53.
5) Four interrupts to MCU.
6) Provide 32 lock registers for software to use to indicate whether
   mailbox is occupied.

[Jassi: Removed unused variable buf_base]

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>