regulator: bd9571mwv: Use "backup_mode" sysfs file instead of "wake_up"

Currently the BD9571MWV PMIC driver uses the standard "wake_up" sysfs
file to control enablement of DDR Backup Mode.

However, configuring DDR Backup Mode is not really equivalent to
configuring the PMIC as a wake-up source.  To avoid confusion, use a
custom "backup_mode" attribute file in sysfs instead.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mark Brown <broonie@kernel.org>