arm64: dts: renesas: ulcb: Add PMIC DDR Backup Power config

On the R-Car Starter Kit Premier/Pro, 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 (SW8) is a momentary switch, hense specify
"rohm,rstbmode-pulse".

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