clk: zx: add clock support to zx296702

It adds a clock driver for zx296702 SoC to register the clock tree to
Common Clock Framework.  All the clocks of bus topology and some the
peripheral clocks are ready with this commit. Some missing leaf clocks
for peripherals will be added later when needed.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>