serial: 8250: Extract platform driver

serial: 8250: Extract platform driver

Extract platform driver to a separate module for better maintenance
and to reduce churn on 8250_core part changes when it's solely related
to the former. No functional changes intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Sunil V L <sunilvl@ventanamicro.com>
Tested-by: Sunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20240506140308.4040735-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>