fvp: Remove unused 'bl2_el_change_mem_ptr' variable

'bl2_el_change_mem_ptr' variable is a left over from the former
BL2/BL3-1 interface.

Change-Id: Ib0979c8e2809e103a41f9c5cc4afec7dd21ac9ab
Merge pull request #125 from sandrine-bailleux/sb/remove-bl2_el_change_mem_ptr

fvp: Remove unused 'bl2_el_change_mem_ptr' variable