Update the documentation about the memory layout on FVP

Update the "Memory layout on FVP platforms" section in the Firmware
Design document to reflect the overlaying of BL1 and BL2 images
by BL3-1 and BL3-2.

Also update the Porting Guide document to mention the
BL31_PROGBITS_LIMIT and BL32_PROGBITS_LIMIT constants.

Change-Id: I0b23dae5b5b4490a01be7ff7aa80567cff34bda8