feat(st): enable MMC_FLAG_SD_CMD6 for SD-cards

This flag allows switching to High-Speed mode on SD-cards.
The gain is ~44ms when using SP_min, and ~55ms with OP-TEE.

Change-Id: Ic396c6a14201580b5e5627e6174b85b437b87cae
Signed-off-by: Yann Gautier <yann.gautier@st.com>
1 file changed