[U-Boot] am33xx config_ddr api change needed to support mDDR

[Markus Brunner]

Hi,
for mDDR to work mddr_sel (bit 28) of ddrctrl->ddrioctrl has to be
set. There is no way of telling config_ddr(unsigned int pll, unsigned
int ioctrl, ...) my desired value, it won't be set atm.