According to the Datasheet :
"All four ports support:
• 1-bit or 4-bit transfer mode specifications for SD and SDIO cards up to
UHS-I SDR104 mode (104 MB/s max) ...".
As for boot mode, please refer to Table 5-7 (SD/eSD Boot Fusemap) and
Table 5-8 (MMC/eMMC Boot Fusemap) of the i.MX6 DQ Reference Manual.
Regards,
Yuri.