docs: memory-devices: convert ti-emif.txt to ReST

Prepare this file to be moved to a kernel book by converting
it to ReST format and renaming it to ti-emif.rst.

While this is not part of any book, mark it as :orphan:, in order
to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>