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>

board/beacon/imx8mm/README

@@ -26,7 +26,7 @@ $ cp firmware-imx-8.5/firmware/ddr/synopsys/lpddr4*.bin $(srctree)
 Build U-Boot
 ============
 $ make imx8mm_beacon_defconfig
-$ make flash.bin ARCH=arm CROSS_COMPILE=aarch64-linux-gnu- ATF_LOAD_ADDR=0x920000
+$ make flash.bin CROSS_COMPILE=aarch64-linux-gnu- ATF_LOAD_ADDR=0x920000
 
 Burn U-Boot to microSD Card
 ===========================

board/logicpd/imx6/README

@@ -17,7 +17,7 @@ Building U-Boot for Logic PD Development Kit
 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

board/technexion/pico-imx7d/README.pico-imx7d_BL33

@@ -4,7 +4,7 @@ The boot sequence is ATF -> OPTEE -> U-Boot -> Linux. U-Boot is in non-secure
 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
 

doc/board/toradex/colibri_imx7.rst

@@ -18,7 +18,6 @@ Build U-Boot
 .. code-block:: bash
 
     $ export CROSS_COMPILE=arm-linux-gnueabi-
-    $ export ARCH=arm
     $ make colibri_imx7_emmc_defconfig # For NAND: colibri_imx7_defconfig
     $ make