clk: mmp2: add 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.

Meaning of the relevant APMU_GPU bits were gotten from James Cameron's
message and [1], the OLPC OS kernel source [2] and Marvell's MMP3 tree.

[1] http://lists.laptop.org/pipermail/devel/2019-April/039053.html
[2] http://dev.laptop.org/git/olpc-kernel/commit/arch/arm/mach-mmp/mmp2.c?h=arm-3.0-wip&id=8ce9f6122

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-13-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>