imx: Remove ARCH= references from documentation

When building U-Boot we select the architecture via Kconfig and not ARCH
being passed in via the environment or make cmdline.

Cc: Adam Ford <aford173@gmail.com>
Cc: Vanessa Maegima <vanessa.maegima@nxp.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Igor Opaniuk <igor.opaniuk@toradex.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/technexion/pico-imx7d/README.pico-imx7d_BL33 b/board/technexion/pico-imx7d/README.pico-imx7d_BL33
index 40324ff..1b34609 100644
--- a/board/technexion/pico-imx7d/README.pico-imx7d_BL33
+++ b/board/technexion/pico-imx7d/README.pico-imx7d_BL33
@@ -4,7 +4,7 @@
 world in this case.
 
 - Build u-boot
-    Set environment variable of CROSS_COMPILE for your toolchain and ARCH=arm
+    Set environment variable of CROSS_COMPILE for your toolchain
     $ make pico-imx7d_bl33_defconfig
     $ make all