I would write like this

_wait_:

while((P0 & 0xF0) == 0xF0) { ; }

key_temp = (P0 & 0xF0) ;

delay(debounce); // debounce delay 50 - 100 mS

if (key_temp == (P0 & 0xF0)) { get_key(); }

while((P0 & 0xF0) != 0xF0) { ; }

if (key_temp) { key_act(); }
key_temp = 0 ;

goto _wait_;

void get_key(void)
{
switch(key_temp)
{
case 0x70:
key_temp = up_key ;
break;
case 0xB0:
key_temp = down_key ;
break;
case 0xD0:
key_temp = left_key;
break;
case 0xE0:
key_temp = right_key;
break;
default :
key_temp = invalid_key;
}
}

Obivously this code and method is not elegant. But ok to start with. This method hoggs the controller.

Alternative is to use say a 50mS timer tick to scan the keys. Note any change of state and look for the changed state in the next loop. If still changed state then a key(s) is pressed so find the key. Else reset the changed state and loop back. The 50mS tick time will provide the debouncing. In this method controller is available for other tasks.