tty: serial: omap: add UPF_BOOT_AUTOCONF flag for DT init

The UPF_BOOT_AUTOCONF flag is needed for proper
flow control support.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>