plat/arm/sgi: add secure memory support for sgi575 and sgiclarka

Remove the platform common plat_arm_security_setup function to allow
platform specific implementations of the security setup function
implemented in the board directory of the platform.

For use by secure software, configure region0 of DMC-620 trustzone
controller to protect the upper 16MB of memory of the first DRAM block
from non-secure accesses.

Change-Id: I9a8c19656702c4fa4f6917b3655b692d443bb568
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>