plat/arm: Fix BL31_BASE when RESET_TO_BL31=1

The value of BL31_BASE currently depends on the size of BL31. This
causes problems in the RESET_TO_BL31 case because the value of
BL31_BASE is used in the model launch parameters, which often changes.

Therefore, this patch fixes BL31_BASE to the middle of Trusted SRAM,
to avoid further model parameter changes in future.

Change-Id: I6d7fa4fe293717d84768974679539c0e0cb6d935
Signed-off-by: David Cunado <david.cunado@arm.com>