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>