openrisc: init: Add support for common clk

When testing the new litex_mmc driver it was found to not work on
OpenRISC due to missing support for common clk.  This patch does the
basic initialization to allow OpenRISC to use the common clk framework.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>