arm64: dts: renesas: Unify the labels for RWDT

The labels for RWDT device node were named as 2 types now:

 - wdt0: r8a7795, r8a7796, r8a77965.
 - rwdt: r8a77970, r8a77990, r8a77995.

To be made consistent, this patch unifis the labels as the hardware
name "rwdt".

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>