When TF-A is built with RESET_TO_BL31=1 option, BL31 is the
first image to be run and should have all the memory allocated
to it except for the memory reserved for Shared RAM at the start
of Trusted SRAM.
This patch fixes FVP BL31 load address and its image size for
RESET_TO_BL31=1 option. BL31 startup address should be set to
0x400_1000 and its maximum image size to the size of Trusted SRAM
minus the first 4KB of shared memory.
Loading BL31 at 0x0402_0000 as it is currently stated in
'\docs\plat\arm\fvp\index.rst' causes EL3 exception when the
image size gets increased (i.e. building with LOG_LEVEL=50)
but doesn't exceed 0x3B000 not causing build error.

Change-Id: Ie450baaf247f1577112f8d143b24e76c39d33e91
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>