plat/st: enable MMC_FLAG_SD_CMD6 for SD-cards

This flag allows switching to High-Speed mode on SD-cards.

Change-Id: Ic396c6a14201580b5e5627e6174b85b437b87cae
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/140773


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>