armv8/ls1043ardb: Add sd boot support

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>
diff --git a/board/freescale/ls1043ardb/README b/board/freescale/ls1043ardb/README
index 4f15557..0556e73 100644
--- a/board/freescale/ls1043ardb/README
+++ b/board/freescale/ls1043ardb/README
@@ -84,3 +84,4 @@
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 a) NOR boot
 b) NAND boot
+c) SD boot