dtoc: Support headers needed for drivers

dtoc: Support headers needed for drivers

Typically dtoc can detect the header file needed for a driver by looking
for the structs that it uses. For example, if a driver as a .priv_auto
that uses 'struct serial_priv', then dtoc can search header files for the
definition of that struct and use the file.

In some cases, enums are used in drivers, typically with the .data field
of struct udevice_id. Since dtoc does not support searching for these,
add a way to tell dtoc which header to use. This works as a macro included
in the driver definition.

Signed-off-by: Simon Glass <sjg@chromium.org>