I have exact the same issue.
I do not find where the above two constants are defined in the u-boot source code.
It looks like a miracle that it compiled fine.
Edit, Ok, according to the definition of MX6_PAD_DECL every thing is clear now.
There exists a definition for MX6_PAD_DECL(KEY_COL0__UART4_TX_DATA.... which is basically creating the constant i was looking for.