Several UART input selects are missing. The fourth input select
for UART2_TX_DATA_ALT0 is actually also missing in the documentation.
(at least in Rev. B of the i.MX 7Dual Reference Manual). However,
when looking at the tables of other input selects, it is very natural
that there must be an input select for the UART2_TX_DATA_ALT0 pad.
The Colibri iMX7 also uses that pad for UART2 RX (in DTE mode), and
it was required to set that particular input select register to get a
working UART2.