mmc: erase: Use TRIM erase when available

The default erase command applies on erase group unit, and
simply round down to erase group size. When the start block
is not aligned to erase group size (e.g. erasing partition)
it causes unwanted erasing of the previous blocks, part of
the same erase group (e.g. owned by other logical partition,
or by the partition table itself).

To prevent this issue, a simple solution is to use TRIM as
argument of the Erase command, which is usually supported
with eMMC > 4.0, and allow to apply erase operation to write
blocks instead of erase group

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>