arm64: dts: renesas: Rename r8a7796* to r8a77960*

CONFIG_ARCH_R8A7796 was renamed to CONFIG_ARCH_R8A77960 in commit
39e57e14 ("soc: renesas: Add ARCH_R8A77960 for existing R-Car
M3-W"), to avoid confusion between R-Car M3-W (R8A77960) and R-Car M3-W+
(R8A77961).

Rename the DTS files as well, for consistency.

Note that DT binding headers, definitions, and compatible values were
not renamed, to preserve backward compatibility.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191211131311.23661-1-geert+renesas@glider.be


Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>