dt-bindings: arm: Document RZ/A2 SoC DT bindings

Add device tree bindings documentation for Renesas RZ/A2 (r7s9210) SoC.
Also document new option for "renesas,bsid"

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>