riscv: dts: thead: th1520: Add IMG BXM-4-64 GPU node

riscv: dts: thead: th1520: Add IMG BXM-4-64 GPU node

Add a device tree node for the IMG BXM-4-64 GPU present in the T-HEAD
TH1520 SoC used by the Lichee Pi 4A board. This node enables support for
the GPU using the drm/imagination driver.

By adding this node, the kernel can recognize and initialize the GPU,
providing graphics acceleration capabilities on the Lichee Pi 4A and
other boards based on the TH1520 SoC.

Add fixed clock gpu_mem_clk, as the MEM clock on the T-HEAD SoC can't be
controlled programatically.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Matt Coster <matt.coster@imgtec.com>
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Signed-off-by: Drew Fustini <fustini@kernel.org>