FVP: Ensure system reset wake-up results in cold boot

platform_get_entrypoint() did not consider that a wakeup due to
System Reset Pin (by reading the power controller's PSYSR) requires
a cold boot. As a result, the code would execute the warm boot path
and eventually panic because entrypoint mailboxes are empty.

This patch ensures that the following wake-up reasons result in cold
boot:
  - Cold Power-on
  - System Reset Pin (includes reset by software)

Fixes ARM-software/tf-issues#217

Change-Id: I65ae0a0f7a46548b575900a5aac107d352b0e2cd