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>
13 jobs for !108 with ag/fixes in 54 minutes and 50 seconds (queued for 4 seconds)
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dco

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freertos-armv5

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linux-seco-imx8mm-c61

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linux-seco-intel-b68

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linux-stm32mp1-av96

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zephyr-96b-avenger

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zephyr-96b-nitrogen

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zephyr-qemu-cortex-m3

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zephyr-qemu-x86

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