video: atmel_hlcdfb: LCDC PWM errata

LCDC PWM is not usable with PWMPS set to DIV_1. Other dividers work fine.
This errata concern sama5d3x devices. It is recommended to use standalone PWM
output for at91sam9g35 and at91sam9x35 devices.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>