s5j/pwm: rework pwm lower-half driver

s5j/pwm: rework pwm lower-half driver

This commit rewrites PWM driver in completely new. Every code lines were
reviewed and then any redundant or unnecessary code lines have been
removed. This reduces binary footprints of PWM driver quite a lot as
following:

    Before:
       text	   data	    bss	    dec	    hex	filename
       1755	    160	    386	   2301	    8fd	s5j_pwm.o

    After:
       text	   data	    bss	    dec	    hex	filename
        655	     72	      0	    727	    2d7	s5j_pwm.o

Change-Id: If2a4e46e47e4c2d7225abfa8419e1ccf76fb08a9
Signed-off-by: Bongryul Lee <bongryul.lee@samsung.com>