Fix resize_t enum name

resize_t is already used in the type support table for RESIZE, rename
the mode enum to resize_mode_t.

Also add a python function to get an enum object by name to the spec
object.

Change-Id: Id12c10929beda469c6e3714518d6e26f18ddd440
Signed-off-by: Kevin Petit <kevin.petit@arm.com>