mfd: bd718x7: Make power button press duration configurable

Allow overwriting the values in BD718XX_REG_PWRONCONFIG0 and
BD718XX_REG_PWRONCONFIG1 via devicetree. Read values in milliseconds and
attempt to round them to something supported by the hardware.

Keep existing values (from bootloader or OTP) if property is not
present.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
[Lee: Fixed-up merge/API conflict]
Signed-off-by: Lee Jones <lee.jones@linaro.org>