drm/imagination: Enable PowerVR driver for RISC-V

drm/imagination: Enable PowerVR driver for RISC-V

Several RISC-V boards feature Imagination GPUs that are compatible with
the PowerVR driver. An example is the IMG BXM-4-64 GPU on the Lichee Pi
4A board. This commit adjusts the driver's Kconfig dependencies to allow
the PowerVR driver to be compiled on the RISC-V architecture.

By enabling compilation on RISC-V, we expand support for these GPUs,
providing graphics acceleration capabilities and enhancing hardware
compatibility on RISC-V platforms.

The RISC-V support is restricted to 64-bit systems (RISCV && 64BIT) as
the driver currently has an implicit dependency on a 64-bit platform.

Add a dependency on MMU to fix a build warning on RISC-V configurations
without an MMU.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://lore.kernel.org/r/20250822-apr_14_for_sending-v13-4-af656f7cc6c3@samsung.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>