pwm: Add LP3943 PWM driver

This is the other of the LP3943 MFD driver.
LP3943 can be used as a PWM generator, up to 2 channels.

* Two PWM generators supported

* Supported PWM operations
  request, free, config, enable and disable

* Pin assignment
  A driver data, 'pin_used' is checked when a PWM is requested.
  If the output pin is already assigned, then returns as failure.
  If the pin is available, 'pin_used' is set.
  When the PWM is not used anymore, then it is cleared.
  It is defined as unsigned long type for atomic bit operation APIs,
  but only LSB 16bits are used because LP3943 has 16 outputs.

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