rockchip: 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.

Acked-by: Kever Yang <kever.yang@rock-chips.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/rockchip/evb_rk3229/README

@@ -22,7 +22,6 @@ Compile the U-Boot
 
   > cd ../u-boot
   > export CROSS_COMPILE=arm-linux-gnueabihf-
-  > export ARCH=arm
   > make evb-rk3229_defconfig
   > make
   > make u-boot.itb

board/rockchip/evb_rk3399/README

@@ -54,7 +54,6 @@ Compile U-Boot
 ==============
 
   > cd ../u-boot
-  > export ARCH=arm64
   > export CROSS_COMPILE=aarch64-linux-gnu-
   > make evb-rk3399_defconfig
   for firefly-rk3399, use below instead:

board/theobroma-systems/lion_rk3368/README

@@ -17,12 +17,12 @@ Configure U-Boot
 Build the TPL/SPL stage
 =======================
 
-  > make CROSS_COMPILE=aarch64-unknown-elf- ARCH=arm
+  > make CROSS_COMPILE=aarch64-unknown-elf-
 
 Build the full U-Boot and a FIT image including the ATF
 =======================================================
 
-  > make CROSS_COMPILE=aarch64-unknown-elf- ARCH=arm u-boot.itb
+  > make CROSS_COMPILE=aarch64-unknown-elf- u-boot.itb
 
 Flash the image
 ===============

board/vamrs/rock960_rk3399/README

@@ -57,7 +57,6 @@ Compile the U-Boot
 
   > cd ../u-boot
   > cp ../rkbin/rk33/rk3399_bl31_v1.00.elf ./bl31.elf
-  > export ARCH=arm64
   > export CROSS_COMPILE=aarch64-linux-gnu-
   > make rock960-rk3399_defconfig
   > make