imx: Remove ARCH= references from documentation

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>