SPM: Combine the same routines in cross call and SVCall

Both Cross and SVC calls require these steps:

- Enter the call
  - Check stack pointer, switch to SPM stack if caller is not
    NS agent.
  - Lock scheduler
  - Verify the return value

- Exit the call
  - Unlock scheduler
  - Trigger PendSV if necessary

This patch renames and reuses the two cross call functions for
SVCall usage.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I7bb924f55fdcbb8c27fa775920a0a69c932df56b
9 files changed