reset: Add optional resets and stubs

This patch adds device_reset_optional and (devm_)reset_control_get_optional
variants that drivers can use to indicate they can function without control
over the reset line. For those functions, stubs are added so the drivers can
be compiled with CONFIG_RESET_CONTROLLER disabled.
Also, device_reset is annotated with __must_check. Drivers ignoring the return
value should use device_reset_optional instead.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Wolfram Sang <wsa@the-dreams.de>