system: Document cpu_physical_memory_*() declarations

Document the following methods use the global address space
and discard success/failure access information:

     - cpu_physical_memory_read()
     - cpu_physical_memory_write()
     - cpu_physical_memory_map()
     - cpu_physical_memory_unmap()

Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20260616020839.19104-2-philmd@oss.qualcomm.com>