SPM: Rename SVC numbers to make more sense

- 'TFM_SVC_HANDLER_MODE' works for SPM initialization.
- 'TFM_SVC_NUMBER_DIVIDER' is actually the end number for
  thread mode supervisor call.
- Define a new name to indicate handler mode SVC number start.

Change-Id: I754b3cb62c892e42b439dc12fe2b3efb0ee72709
Signed-off-by: Ken Liu <Ken.Liu@arm.com>