accel/amdxdna: Add documentation for AMD NPU accelerator driver

AMD NPU (Neural Processing Unit) is a multi-user AI inference accelerator
integrated into AMD client APU. NPU enables efficient execution of Machine
Learning applications like CNN, LLM, etc. NPU is based on AMD XDNA
Architecture. NPU is managed by amdxdna driver.

Co-developed-by: Sonal Santan <sonal.santan@amd.com>
Signed-off-by: Sonal Santan <sonal.santan@amd.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241118172942.2014541-2-lizhi.hou@amd.com