ARM: shmobile: Consolidate the pm code for R-Car Gen2

The pm code for R-Car Gen2 is scatterd in each SoC. These files
(pm-r8a7790.c/pm-r8a7791.c) have some overlap code. This change
consolidate the pm code for R-Car Gen2 into one.

Signed-off-by: Gaku Inami <gaku.inami.xw@bp.renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>