fvp: Properly detect the location of BL1 R/W data

fvp: Properly detect the location of BL1 R/W data

There was already a rudimentary mechanism to detect whether BL1
R/W data was loaded at the top or bottom of memory. Basically,
 - either BL1 was loaded at the very end of the trusted RAM
 - in all other cases BL1 was considered sitting at the bottom of
   the memory and the memory usage structure was updated accordingly,
   potentially resulting in critical memory waste.
For instance, if BL1 R/W base address was set to
(TZRAM_END - 4096 - bl1_size), it would virtually occupy the whole
memory.

This patch improves the mechanism to detect the location of BL1
to avoid such scenarios.

Change-Id: I224a9edf0fe8d34208545d84b28b63f2bb830d03