dt-bindings: serial: Add renesas,em-uart bindings

Document Device Tree bindings for the Renesas EMMA Mobile UART
Interface.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Rob Herring <robh@kernel.org>