amlogic: 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: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Cc: u-boot-amlogic@groups.io
Signed-off-by: Tom Rini <trini@konsulko.com>