regulator: 88pg86x: add DT bindings document

The only device-specific node names are "buck1" and "buck2" for the two
regulators present on the device. Sleep mode GPIO and per-regulator GPIO
enable pins are not exposed (the driver does not support them either).

Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Mark Brown <broonie@kernel.org>