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/logicpd/imx6/README b/board/logicpd/imx6/README
index 26d053a..19f79c5 100644
--- a/board/logicpd/imx6/README
+++ b/board/logicpd/imx6/README
@@ -17,7 +17,7 @@
 To build U-Boot for the Dual and Quad variants:
 
  make imx6q_logic_defconfig
- make u-boot.imx ARCH=arm CROSS_COMPILE=arm-linux-
+ make u-boot.imx CROSS_COMPILE=arm-linux-
 
 
 Flashing U-Boot into the SD card