uboot-imx

README.fsl-ddr typos and update to reflect hotkey

Documentation fix to README.fsl-ddr to fix typos and
to reflect use of 'd' hotkey to enter the FSL DDR debugger.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>