Add WFI in platform's unexpected error handlers

This patch adds a WFI instruction in the default implementations of
plat_error_handler() and plat_panic_handler(). This potentially reduces
power consumption by allowing the hardware to enter a low-power state.
The same change has been made to the FVP and Juno platform ports.

Change-Id: Ia4e6e1e5bf1ed42efbba7d0ebbad7be8d5f9f173