gpio: Support ROHM BD71815 GPOs

Support GPO(s) found from ROHM BD71815 power management IC. The IC has two
GPO pins but only one is properly documented in the data-sheet. The driver
exposes by default only the documented GPO. The second GPO is connected to
E5 pin and is marked as GND in the data-sheet. Control for this
undocumented pin can be enabled using a special DT property.

This driver is derived from work by Peter Yang <yanglsh@embest-tech.com>
although not so much of the original is left.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>