rpi3: Enable SDHost driver in BL2

This patch inits SDHost in BL2 earlysetup. BL2 can start operating mmc
commands to read/write MMC raw blocks.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
diff --git a/plat/rpi3/include/platform_def.h b/plat/rpi3/include/platform_def.h
index 69ebdb3..4d90222 100644
--- a/plat/rpi3/include/platform_def.h
+++ b/plat/rpi3/include/platform_def.h
@@ -244,6 +244,8 @@
 #define MAX_IO_DEVICES			U(3)
 #define MAX_IO_HANDLES			U(4)
 
+#define MAX_IO_BLOCK_DEVICES		U(1)
+
 /*
  * Serial-related constants.
  */