mmc: Define a constant for the maximum block size

The number 512 appears quite a bit in the mmc code. Add a constant for this
so that it can be used here and in other parts of the code (e.g. SPL code
which loads from mmc).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@google.com>
2 files changed