mx51/iomux: add UART and GPIO pad definitions for imx51_3ds board

Add UART2 CTS/RTS and UART3 RXD/TXD pad definitons for imx51_3ds board,
add GPIO_1_6 definiton because this pin is used as a CPLD parent irq
request pin on imx51_3ds board.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>