dt-bindings: regulator: fixed: add off-on-delay-us property

When disabling a fixed regulator, it may take some time to let the
voltage drop to the expected value, such as zero. If not delay
enough time, the regulator might have been always enabled.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1572311875-22880-2-git-send-email-peng.fan@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>