According section
    8.2.5.313 Select Input Register (IOMUXC_UART1_RXD_SELECT_INPUT)
of 
    i.MX 8M Mini Applications Processor Reference Manual, Rev. 3, 11/2020
the required setting for this specific pin configuration is "1"
Signed-off-by: Pierluigi Passaro <pierluigi.p@variscite.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Fixes: c1c9d413 ("dt-bindings: imx: Add pinctrl binding doc for imx8mm")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>