powerpc: 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: Po Liu <po.liu@nxp.com>
Cc: Qiang Zhao <qiang.zhao@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>