arm64: dts: r8a7795: Use SYSC "always-on" PM Domain for RWDT node

Hook up the RWDT device node to the SYSC "always-on" PM Domain, for a
more consistent device-power-area description in DT.

Cfr. commit 38dbb45e ("arm64: dts: r8a7795: Use SYSC "always-on" PM
Domain")

Fixes: f43838a7ae014cba ("arm64: dts: r8a7795: Add RWDT node")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>