The SMDK board uses LT3591 as backlight LED driver of LTE480WV LCD.
According to the LT3591 datasheet, the switching frequency should
be 1MHz. So, PWM period is calculated by following formula:

PWM period = 1/switching frequency
	   = 1/1MHz
	   = 1000ns

Thus, the PWM backlight period should be 1000ns.
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>