mmc: remove dw_mmc-zx driver

The zte zx platform is getting removed, so this driver is no
longer needed.

Cc: Jun Nie <jun.nie@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210120142801.334550-2-arnd@kernel.org


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