This patch updates the user guide instructions for
RESET_TO_SP_MIN and RESET_TO_BL31 cases. The load
address for BL31 had to be updated because of increase
in code size. Also, information about PIE support when
RESET_TO_BL31=1 for FVP is added.

In the case of RESET_TO_SP_MIN, the RVBAR address
was wrong in the instruction. This is also corrected
in the patch.

Change-Id: I65fe6d28c5cf79bee0a11fbde320d49fcc1aacf5
Signed-off-by: Soby Mathew <soby.mathew@arm.com>