Fix "WARNING: document isn't included in any toctree" by adding
the lemp9 to mainboard/index.md.
Change-Id: Id6d8f9e2aab6dc7ad4baf1b37d88e531acb757f4
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39800
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
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