ARM: dts: imx6sx: fix the pad QSPI1B_SCLK mux mode for uart3

The pad QSPI1B_SCLK mux mode 0x1 is for function UART3_DTE_TX,
correct the mux mode.

Fixes: 743636f2

 ("ARM: dts: imx: add pin function header for imx6sx")
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>