mmc: stm32_sdmmc2: manage CMD6

For SD-cards, CMD6 is used to switch functions, like setting high speed
mode. As it has another meaning for eMMC, and may not work on standard
capacity SD-cards, it must be checked with MMC_IS_SD_HC flag.
As ACMD6 is also used, and will have the same index, a check on CMD/ACMD
commands is done: a boolean is stored depending on previous command. It is
set to true if CMD55 is issued, for other commands it is set to false.

Change-Id: I6c2b9c7637656f858601ec075de1cb5f57af271a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/135782


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>