soc: renesas: rcar-sysc: Add R8A77961 support

Add support for the power areas in the Renesas R-Car M3-W+ (R8A77961)
SoC to the R-Car System Controller driver.

R-Car M3-W+ (aka R-Car M3-W ES3.0) is very similar to R-Car
M3-W (R8A77960), which allows for both SoCs to share a driver:
  - R-Car M3-W+ lacks the A2VC power area, so its area must be
    nullified,
  - The existing support for the SYSCEXTMASK register added in commit
    9bd645af9d2a49ac ("soc: renesas: r8a7796-sysc: Fix power request
    conflicts") applies to ES3.0 and later only.
    As R-Car M3-W+ uses a different compatible value, differentiate
    based on that, instead of on the ES version.

Based on a patch in the BSP by Dien Pham <dien.pham.ry@renesas.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-7-geert+renesas@glider.be