arm64: dts: renesas: r8a77980: use CPG core clock macros

Now that the commit 35b3c462

 ("dt-bindings: clock: add R8A77980 CPG
core clock definitions") has hit Linus' tree, we can replace the bare
numbers (we had to use to avoid a cross tree dependency) with these macro
definitions...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>