arm64: dts: renesas: ebisu: Add PMIC DDR0 Backup Power config

On Ebisu the DDR0 power rail needs to be kept powered when backup mode is
enabled.  Reflect this in the "rohm,ddr-backup-power" property for the
BD9571MWV PMIC node.

The accessory power switch (SW23) is a toggle switch, hence specify
"rohm,rstbmode-level".

Based on advice from Geert Uytterhoeven.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>