imx: add necessary lpuart console_flush callback for debug

Current lpuart driver does NOT implement .console_flush callback,
if debug console is enabled, the console_flush() will call the
undefined .console_flush callback(NULL) for lpuart and leak to
panic, this patch adds .console_flush callback to make lpuart work
for debug mode.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>