In ltc430x(4), add the ability to configure control register 2 via FDT or

In ltc430x(4), add the ability to configure control register 2 via FDT or
hints data.  Control register 2 holds the settings a user might want to
configure, such as the timeout value for idle busses and whether to enable
the mass-writes feature.

Also add hint support for disconnecting idle busses (which was already
supported using FDT data).

Update the manpage with the new features, and also split the hints section
into separate lists of required and optional hints.