dt-bindings: power: renesas,rzg2l-sysc: Add DT binding documentation for SYSC controller

Add DT binding documentation for SYSC controller found on
RZ/G2{L,LC,UL} SoC's.

SYSC block contains the LSI_DEVID register which is used to retrieve
SoC product information.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20210609163717.3083-2-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>