Platform/Sgi: clear the error injection instructions memory region

The error injection intructions that is pointed to by the EINJ table are
placed in a region of memory reserved for it. Clear this memory region
before it is used.

Change-Id: I1aac512045372ac93cf1a5e2a8d670c5df7a523c
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>