Add SP804 delay timer support to the FVP BSP.

This commit simply provides the 3 constants needed by the SP804
delay timer driver and calls sp804_timer_init() in
bl2_platform_setup(). The BSP does not currently use the delay
timer functions.

Note that the FVP SP804 is a normal world accessible peripheral
and should not be used by the secure world after transition
to the normal world.

Change-Id: I5f91d2ac9eb336fd81943b3bb388860dfb5f2b39
Co-authored-by: Dan Handley <dan.handley@arm.com>