clk: mpc512x: add backwards compat to the CCF code

extend the recently added COMMON_CLK platform support for MPC512x such
that it works with incomplete device tree data which lacks clock specs

Cc: Mike Turquette <mturquette@linaro.org>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
[agust@denx.de: moved node macro definitions out of the function body]
Signed-off-by: Anatolij Gustschin <agust@denx.de>