rockchip: Update todo in README.rockchip

MMC support works now, so it can be dropped from the todo
Signed-off-by: NSjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NSimon Glass <sjg@chromium.org>

doc/README.rockchip

@@ -161,7 +161,6 @@ Future work
 
 Immediate priorities are:
 
-- MMC support (in U-Boot itself)
 - GPIO (driver exists but is lightly tested)
 - I2C (driver exists but is non-functional)
 - USB host