serial: samsung: Add samsung_early_read to support early kgdboc

The 'kgdboc_earlycon_init' looks for boot console that has both .read
and .write callbacks. Adds 'samsung_early_read' to samsung_tty.c's early
console to support kgdboc.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Woody Lin <woodylin@google.com>
Link: https://lore.kernel.org/r/20220302114923.144523-1-woodylin@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>