Hi,
I add 2 lines and it works!
//You should add two lines before configure PD6 and PD7
HWREG(GPIO_PORTD_BASE + 0x520) = 0x4C4F434B; //Unlock GPIOCR, see datasheet page 711
HWREG(GPIO_PORTD_BASE + 0x524) = 0x000000ff; //Change GPIOCR from 0x7f-> 0xff,see datasheet page 712
Goodluck!
Le Manh Hai