dfu: mmc: Add support for exposing whole mmc device

Add support for exposing the whole mmc device by setting the 'size'
parameter to 0. This can be useful in case it is not clear what the
total device size is up front. Update the documentation accordingly.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Link: https://lore.kernel.org/r/20231029223740.284149-1-marex@denx.de


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>