mmc: usdhi6rol0: Add maintainers

The usdhi6rol0 driver is exclusively used for the ARTPEC family of
SoCs. Other SoCs with the same IP of Panasonic origin use the tmio_mmc
driver. Therefore we assigner maintainer responsibility to us at Axis
until the two drivers become unified.

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>