rockchip: Update todo in README.rockchip

MMC support works now, so it can be dropped from the todo

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.rockchip b/doc/README.rockchip
index 347fc05..feb11ff 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -161,7 +161,6 @@
 
 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