docs: mmc: update maintainer name and URL

The mmc-utils repo is no longer in /cjb/ and Ulf has taken over
maintenance.

Signed-off-by: Andy Isaacson <adi@hexapodia.org>
Link: https://lore.kernel.org/r/20211027230505.GA23994@hexapodia.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>