regulator: bd9571mwv: Add support for toggle power switches

Extend the existing support for backup mode to toggle power switches.
With a toggle power switch (or level signal), the following steps must
be followed exactly:
   1. Configure PMIC for backup mode, to change the role of the
      accessory power switch from a power switch to a wake-up switch,
   2. Switch accessory power switch off, to prepare for system suspend,
      which is a manual step not controlled by software,
   3. Suspend system,
   4. Switch accessory power switch on, to resume the system.

Hence the PMIC is configured for backup mode when "on" or "1" is written
to the PMIC's "backup_mode" virtual file in sysfs.  Conversely, writing
"off" or "0" reverts the role of the accessory switch to a power
switch.

Unlike with momentary switches, backup mode is not enabled by default,
as enabling it prevents the board from being powered off using the power
switch, which may confuse the user.

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>