Azure: Add excludes to the imx8_imx9 job

The job to build all imx8 and imx9 platforms is currently close to, or
sometimes exceeding the allowed build time. Exclude some platforms that
are already being built under their vendor-specific job as well to
reduce the time.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>