imx8: update README

After u-boot.cnt is padded to flash.bin automatically by script,
no need to burn the image mannually, so drop the step.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8qm_mek/README b/board/freescale/imx8qm_mek/README
index ebf630c..a187ad8 100644
--- a/board/freescale/imx8qm_mek/README
+++ b/board/freescale/imx8qm_mek/README
@@ -41,7 +41,6 @@
 ============
 $ make imx8qm_mek_defconfig
 $ make flash.bin
-$ dd if=u-boot.cnt of=flash.bin bs=512 seek=1984
 
 Flash the binary into the SD card
 =================================
diff --git a/board/freescale/imx8qxp_mek/README b/board/freescale/imx8qxp_mek/README
index 1ee0357..e676e88 100644
--- a/board/freescale/imx8qxp_mek/README
+++ b/board/freescale/imx8qxp_mek/README
@@ -41,7 +41,6 @@
 ============
 $ make imx8qxp_mek_defconfig
 $ make flash.bin
-$ dd if=u-boot.cnt of=flash.bin bs=512 seek=1984
 
 Flash the binary into the SD card
 =================================