powerpc/mpc8xxx: Enable entering DDR debugging by key press

Using environmental variable "ddr_interactive" to activate interactive DDR
debugging seomtiems is not enough. For example, after updating SPD with a
valid but wrong image, u-boot won't come up due to wrong DDR configuration.
By enabling key press method, we can enter debug mode to have a chance to
boot without using other tools to recover the board.

CONFIG_FSL_DDR_INTERACTIVE needs to be defined in header file. To enter the
debug mode by key press, press key 'd' shortly after reset, like one would
do to abort auto booting. It is fixed to lower case 'd' at this moment.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>