serial: pch_uart: add console poll support

Implement console poll for pch_uart, this could enable KGDBoC when
on pch-uart console.

Signed-off-by: Liang Li <liang.li@windriver.com>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>