arch: nios2: Enable the common clk subsystem on Nios2

This patch adds support for common clock framework on Nios2. Clock
framework is commonly used in many drivers, and this patch makes it
available for the entire architecture, not just on a per-driver basis.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>