tty: serial: samsung_tty: loopback mode support

tty: serial: samsung_tty: loopback mode support

Internal loopback mode can be supported by setting
UCON register's Loopback Mode bit. The mode & bit can be supported
since s3c2410 and later SoCs. The prefix of LOOPBACK / BIT(5) naming
should be also changed to S3C2410_ in order to avoid confusion.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Link: https://lore.kernel.org/r/20220629004141.51484-1-chanho61.park@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>