x86: baytrail: Add option to disable the internal UART to setup_early_uart()

This patch adds a parameter to the function setup_early_uart() to either
enable or disable the internal BayTrail legacy UART. Since the name
setup_early_uart() does not match its functionality any more, lets
rename it to setup_internal_uart() as well in this patch.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>