linux-yocto/5.10: fix SERIAL_OF_PLATFORM warning

Integrating the following commit(s) to linux-yocto.:

    a0249ac3bae cfg: classify CONFIG_SERIAL_OF_PLATFORM as non-hardware

By declaring the option non-hardware, we make it optional and hence
fix the warning on non OF platforms.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>