Fix LENGTH attribute value in linker scripts

This patch fixes the incorrect value of the LENGTH attribute in
the linker scripts. This attribute must define the memory size, not
the limit address.

Fixes ARM-software/tf-issues#252

Change-Id: I328c38b9ec502debe12046a8912d7dfc54610c46
Merge pull request #212 from jcastillo-arm/jc/tf-issues/252

Fix LENGTH attribute value in linker scripts