MAINTAINERS: Add Lee Jones as reviewer for the PWM subsystem

Lee has kindly offered his help in sharing the patch review workload for
the PWM subsystem. If this works out well between Lee, Uwe and myself it
may be a good idea to maintain the subsystem as a group.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>