video: atmel: implement lcd_setcolreg function

implement the common api lcd_setcolreg in include/lcd.h

Signed-off-by: Bo Shen <voice.shen@atmel.com>
[agust: fixed commit log and gcc 4.6 -Wparentheses warnings]
Signed-off-by: Anatolij Gustschin <agust@denx.de>