odroid-c2: README: MMC is supported

Mention eMMC and microSD as supported devices.

They have been enabled with patch
d0c5c8d529f16fa88ab52a3b5dd2d4fc03664f19
odroid-c2: enable new Meson GX MMC driver in board defconfig
which was accepted for u-boot-mmc.git.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/board/amlogic/odroid-c2/README b/board/amlogic/odroid-c2/README
index d6d266a..b407c04 100644
--- a/board/amlogic/odroid-c2/README
+++ b/board/amlogic/odroid-c2/README
@@ -18,6 +18,7 @@
 
 Currently the u-boot port supports the following devices:
  - serial
+ - eMMC, microSD
  - Ethernet
 
 u-boot compilation