serial: 16550: Add getfcr accessor

Add function which allows fetching the default FCR register setting
from platform data for DM , while retaining old behavior for non-DM
by returning UART_FCRVAL.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>