ARM: dts: artpec: update clock bindings in artpec6.dtsi

The clock binding for the main clock controller was changed to an
indexed controller style binding on request of the clk
maintainers. This updates the dtsi to use the new bindings.

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>