pwm: Take over maintainership of the PWM subsystem

I'm taking over the maintainership of the PWM subsystem. This commit
also adds the URLs to the gitorious project and repository as well as
any missing files related to the PWM subsystem.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>