clk: axi-clkgen: Add support for v2

This patch adds support for the new v2 version of the axi-clkgen core.
Unfortunately the method of accessing the registers is quite different on v2,
while the content still stays largely the same. So the patch adds a small
abstraction layer which implements the specific read and write functions for v1
and v2 in callback functions.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

Documentation/devicetree/bindings/clock/axi-clkgen.txt

@@ -5,7 +5,7 @@ This binding uses the common clock binding[1].
 [1] Documentation/devicetree/bindings/clock/clock-bindings.txt
 
 Required properties:
-- compatible : shall be "adi,axi-clkgen".
+- compatible : shall be "adi,axi-clkgen-1.00.a" or "adi,axi-clkgen-2.00.a".
 - #clock-cells : from common clock binding; Should always be set to 0.
 - reg : Address and length of the axi-clkgen register set.
 - clocks : Phandle and clock specifier for the parent clock.