clk: Hi6220: add stub clock driver

On Hi6220, there have some clocks which can use mailbox channel to send
messages to power controller to change frequency; this includes CPU, GPU
and DDR clocks.

For dynamic frequency scaling, firstly need write the frequency value to
SRAM region, and then send message to mailbox to trigger power controller
to handle this requirement. This driver will use syscon APIs to pass SRAM
memory region and use common mailbox APIs for channels accessing.

This init driver will support cpu frequency change firstly.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>