x86: acpi: Let OS know that console already had been initialized

x86: acpi: Let OS know that console already had been initialized

SPCR has no clue if the UART base clock speed is different to
the default one. However, the SPCR 1.04 defines baud rate 0 as
a preconfigured state of UART and OS is supposed not to touch
the configuration of the serial device.

Linux kernel supports that starting from v5.0, see commit
b413b1abeb21 ("ACPI: SPCR: Consider baud rate 0 as preconfigured state")
for the details.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>