The primary usage of `RUN_IMAGE` SMC function id, used by BL2 is to
make a request to BL1 to execute BL31. But BL2 also uses it as
opcode to check if it is allowed to execute which is not the
intended usage of `RUN_IMAGE` SMC.

This patch removes the usage of `RUN_IMAGE` as opcode passed to
next EL to check if it is allowed to execute.

Change-Id: I6aebe0415ade3f43401a4c8a323457f032673657