arm64: dts: r8a7795: Add PSCI node

Add PSCI node for r8a7795 SoC, and cpu node enable-method property is
set to "psci".

Signed-off-by: Gaku Inami <gaku.inami.xw@bp.renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>