bus: imx-weim: add DT overlay support for WEIM bus

Add OF reconfiguration notifier handler for WEIM bus to setup Chip
Select timings on runtime creation of child devices.

However, it is not possible to load another DT overlay with conflicting
CS timings with previously loaded overlay, even if the first one is
unloaded. The reason is that there is no acces to CS timing property of
a device node being removed, thus we can't track which of configured CS
are available for re-configuration.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>