linux-intel: Fix CONFIG_SERIAL_OF_PLATFORM build warning on 5.10

Due to a configuration bug in upstream, the build system outputs a
configuration warning similar to:

---------- CONFIG_SERIAL_OF_PLATFORM -----------------
Config: CONFIG_SERIAL_OF_PLATFORM
From:
[...]/kernel-source/.kernel-meta/configs/v5.10/standard/cfg/8250.cfg
Requested value:  CONFIG_SERIAL_OF_PLATFORM=y
Actual value:

Avoid this by disabling the specific config.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>