dt-bindings: arm: realtek: Document RTD1619 and Realtek Mjolnir EVB

Define compatible strings for Realtek RTD1619 SoC and Realtek Mjolnir EVB.

Signed-off-by: James Tai <james.tai@realtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>