[U-Boot] Improper assumption of serial port

[Vladimir 'φ-coder/phcoder' Serbinenko]

U-boot on coreboot assumes that serial port is always available and
improperly interprets missing ones as stream of 0xff. What would be the
proper way to handle this? Is it acceptable to probe at it's address
whether we have a valid ns8250/ns16550 device (we can read back the
divisor and compare with what we have set) and dropping the serial if
none is available?

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