dt-bindings: arm: sunxi: add Elimo bindings

Document board compatible names for Elimo Engineering Impetus and Initium

Signed-off-by: Matteo Scordino <matteo.scordino@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201105183231.12952-3-matteo.scordino@gmail.com