scripts/dts/gen_defines.py: generate initializers for phandle/val lists

When a phandle-array (compound) has multiple members generate a define
that says how many there are; generate an initializer for each
individual member; and generate an initializer for the sequence of
members.

This allows drivers that expect multiple values in a compound to process
them without horrendous conditional compilation code attempting to
detect the number of elements in the compound.

It also eliminates the need to repeat the long prefix when initializing
a structure with the fields of a single compound.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>