pwm: add PWM driver on CLV platform

pwm: add PWM driver on CLV platform

BZ: 25800

On Clovertrail platform, Pulse Width Modulator driver needs to be
supported to control the LED brightness and etc. This patch adds
this driver and changes the interface in LED driver.

Change-Id: I6f7595188e4b3f632e2d1551c9f85715aa1d4b04
Signed-off-by: Li, Ning <ning.li@intel.com>
Reviewed-on: http://android.intel.com:8080/37161
Reviewed-by: Yang, Bin <bin.yang@intel.com>
Reviewed-by: Du, Alek <alek.du@intel.com>
Tested-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>