docs: hwmon: aspeed-pwm-tacho: convert to ReST format

Convert aspeed-pwm-tacho to ReST format, in order to allow it to
be parsed by Sphinx.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>