odroid-c2: README: MMC is supported

Mention eMMC and microSD as supported devices.

They have been enabled with patch
d0c5c8d5
odroid-c2: enable new Meson GX MMC driver in board defconfig
which was accepted for u-boot-mmc.git.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

board/amlogic/odroid-c2/README

@@ -18,6 +18,7 @@ Schematics are available on the manufacturer website.
 
 Currently the u-boot port supports the following devices:
  - serial
+ - eMMC, microSD
  - Ethernet
 
 u-boot compilation