ARM: dts: imx6: phyFLEX: Fix UART hardware flow control

Serial interface uart3 on phyFLEX board is capable of 5-wire connection
including signals RTS and CTS for hardware flow control.

Fix signals UART3_CTS_B and UART3_RTS_B padmux assignments and add
missing property "uart-has-rtscts" to allow serial interface to be
configured and used with the hardware flow control.

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>