Include a 33 Ω series resistor on MMCx_CLK (as close to the processor as possible). This signal is used as an input on read transactions. The resistor eliminates possible signal reflections on the signal that can cause false clock transitions.
This also requires you to set RXACTIVE=1 in the pinmux configuration for the MMC_CLK signal.
When connecting a device (card or eMMC), include 10k pullups on RST#, CMD, and all DAT signals.