docs: bus-devices: ti-gpmc.rst: convert it to ReST

docs: bus-devices: ti-gpmc.rst: convert it to ReST

In order to be able to add this file to a book, it needs
first to be converted to ReST and renamed.

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>