arm64: dts: imx8mm: Correct SAI3 RXC/TXFS pin's mux option #1

According to i.MX8MM reference manual Rev.1, 03/2019:

SAI3_RXC pin's mux option #1 should be GPT1_CLK, NOT GPT1_CAPTURE2;
SAI3_TXFS pin's mux option #1 should be GPT1_CAPTURE2, NOT GPT1_CLK.

Fixes: c1c9d413

 ("dt-bindings: imx: Add pinctrl binding doc for imx8mm")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>