io_block: define MAX_IO_BLOCK_DEVICES as unsigned

This is used as a table index, and already compared with an unsigned int:
block_dev_count.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/hisilicon/hikey/include/platform_def.h b/plat/hisilicon/hikey/include/platform_def.h
index f2d358a..485eb38 100644
--- a/plat/hisilicon/hikey/include/platform_def.h
+++ b/plat/hisilicon/hikey/include/platform_def.h
@@ -41,7 +41,7 @@
 #define MAX_IO_DEVICES			3
 #define MAX_IO_HANDLES			4
 /* eMMC RPMB and eMMC User Data */
-#define MAX_IO_BLOCK_DEVICES		2
+#define MAX_IO_BLOCK_DEVICES		U(2)
 
 /* GIC related constants (no GICR in GIC-400) */
 #define PLAT_ARM_GICD_BASE		0xF6801000