docs: serial: convert docs to ReST and rename to *.rst

The converted files are focused at the Kernel internal API,
so, this is a good candidate for the kernel API set of books.

The conversion is actually:
  - add blank lines and identation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_core.h

@@ -45,7 +45,7 @@ struct device;
 
 /*
  * This structure describes all the operations that can be done on the
- * physical hardware.  See Documentation/serial/driver for details.
+ * physical hardware.  See Documentation/serial/driver.rst for details.
  */
 struct uart_ops {
 	unsigned int	(*tx_empty)(struct uart_port *);