mmc: meson-gx: add CMD23 mode

CMD23 mode (use "set block count" command before transferring multiple
data blocks) typically is more performant as host / card know upfront
how many data blocks to expect. Therefore add support for this mode to
the driver.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>