imx8mq_evk/README: remove ARCH environment variable

There is no need to set the ARCH variable when building U-Boot. In fact,
the ARCH name in U-Boot is 'arm'.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
diff --git a/board/freescale/imx8mq_evk/README b/board/freescale/imx8mq_evk/README
index 07dbfb0..1da6eaf 100644
--- a/board/freescale/imx8mq_evk/README
+++ b/board/freescale/imx8mq_evk/README
@@ -23,7 +23,6 @@
 
 Build U-Boot
 ====================
-$ export ARCH=arm64
 $ export CROSS_COMPILE=aarch64-poky-linux-
 $ make imx8mq_evk_defconfig
 $ make flash.bin