Pointer properties can be set only by code, not by device_add.  A
device with a pointer property can work with device_add only when the
property may remain null.

This is the case for property "interrupt_vector" of device
"etraxfs,pic".  Add a comment there.

Set cannot_instantiate_with_device_add_yet for the other devices with
pointer properties, with a comment explaining why.

Juha Riihimäki and Peter Maydell deserve my thanks for making "pointer
property must not remain null" blatantly obvious in the OMAP devices.

Only device "smbus-eeprom" is actually changed.  The others are all
sysbus devices, which get cannot_instantiate_with_device_add_yet set
in their abstract base's class init function.  Setting it again in
their class init function is technically redundant, but serves as
insurance for when sysbus devices become available with device_add,
and as documentation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com> (for ETRAX)
Signed-off-by: Andreas Färber <afaerber@suse.de>