dtc: Abolish asize field of struct data

The asize field in struct data is a hangover from the early days when
a struct data was sometimes allowed to refer to a static chunk of
memory rather than a malloc()ed block.

That's long gone, since the lifetime issues were far more trouble than
it was worth, so get rid of the asize field.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>