dt-bindings: marvell,mmp2: Add clock ids for the GPU clocks

MMP2 has a single GC860 core while MMP3 has a GC2000 and a GC300.
On both platforms there's an AXI bus interface clock that's common for
all GPUs and each GPU core has a separate clock.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-12-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>