XTFPGA boards should populate core memory regions the same way sim
machine does. Move xtensa_create_memory_regions implementation to a
separate file and use it to create instruction and data memory regions
on XTFPGA boards.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
While at it rename lx60 (named after the first board of the family) to
more generic xtfpga (the family name).
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>