What about the phrase in the Technical Reference Manual (SPRUH73I):
26.1.7.5.4 Initialization and Detection
The ROM Code initializes the memory device or card connected on MMC interface using the standard
High-Voltage range (3.3 V).
In the table shown in the answer above booting an AM335x using MMC0-1 is supported for both 1.8V and 3.3V.
Which is right?