mmc: Avoid HS400 mode when accessing boot partitions

U-Boot code currently only applies this restriction to HS200 mode,
extend this to HS400 mode as well.

Currently U-Boot code not support accessing boot partition in HS200/400
mode. This needs more check.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>