imx8mq_pins: fix configuration for UART4 on ECSPI2 pads

When routing UART4 using the ECSPI2 pads, register
IOMUXC_UART4_RXD_SELECT_INPUT (offset 0x050C) should be changed only
when dealing with RX, as its name suggests.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>