clk: add artpec-6 clock controller

Add a driver for the main clock controller of the Artpec-6 Soc.

Signed-off-by: Lars Persson <larper@axis.com>
[sboyd@codeaurora.org: Reformatted driver structure and of match]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>