ARM: dts: imx53: Update UART configuration on M53Menlo

Enable flow control lines on UART1 and UART2, add matching pinmux entries.
Add and enable UART3 with RS485 mode enabled on boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
To: linux-arm-kernel@lists.infradead.org
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>