arm64: dts: renesas: salvator-common: Add PMIC DDR Backup Power config

On Salvator-X(S), all of the DDR0, DDR1, DDR0C, and DDR1C power rails
need 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".

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