x86: cpu: add docstring to scu_ipc_command()

These comments were copied from the Linux kernel driver in
drivers/platform/x86/intel_scu_ipc.c

Signed-off-by: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>