dt-bindings: mmc: Correct typo "upto" to "up to"

The word "upto" is a typo for "up to". Correct this typo in the
mmc-controller-common binding documentation.

Signed-off-by: Akiyoshi Kurita <weibu@redadmin.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>