Makefile: Fix the location of the migration file

Since commit e1910d93 ("doc: driver-model: Convert MIGRATION.txt to
reST") MIGRATION.txt has been converted to migration.rst, so update
the Makefile references accordingly.

Fixes: e1910d93 ("doc: driver-model: Convert MIGRATION.txt to reST")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>