Have you included a series resistor on the clock to dampen reflections? Include a 22-Ω series resistor on MMCx_CLK (as close to the processor as possible). This signal is used as an input on read transactions, and the resistor eliminates possible signal reflections on the signal, which can cause false clock transitions.
Have you included pull-up resistors for SD-CARD signals to the proper I/O voltage rail? Pull-up resistors are needed on all data and command signals. These pull-ups should be 10K Ω for SD-CARD implementations and 49.9K Ω for embedded device connections, such as eMMC memory devices. The pull-up resistors should be connected to the same supply used for the MMC I/O rail, VDDSHV6 for MMC0 or VDDSHV7 for MMC1.
Are you supporting UHS-I using the internal SDIO LDO? The AM65x provides an internal SDIO LDO that has a switchable output to support either 3.3-V I/O or 1.8-V I/O. The switchable output must be connected to the proper I/O voltage rail and the bias voltage for the MMC interface used, VDDSHV6 and VDDS6 for MMC0 or VDDSHV7 and VDDS7 for MMC1. The proper capacitance value must also be included for proper operation of the LDO. In addition, a 270-Ω shunt resistor must be included between CAP_VDDSHV_SDIO and ground.
Have you connected the SD-CARD’s VDD signal to a 3.3-V I/O supply? While the I/O voltage for the SD-CARD interface can be either 1.8 V or 3.3 V, the SD card's VDD signal should be connected to a fixed 3.3-V rail. In other words, the card's VDD must remain at 3.3 V even for the UHS-I modes of operation. Only the signaling levels change in these modes, not VDD.