feat(timer): support PPI timer interrupts

Add support for timers that use a PPI as timer interrupt. A new
function, tftf_initialise_timer_secondary_core(), is added to initialize
the PPI on the secondary CPUs from tftf_warm_boot_main().

Change-Id: Ia343ce10b0b51e72b9e520b1fab0ea7ba0a43f2c
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/include/lib/timer.h b/include/lib/timer.h
index 0bfff01..ff57cbd 100644
--- a/include/lib/timer.h
+++ b/include/lib/timer.h
@@ -32,6 +32,8 @@
  */
 int tftf_initialise_timer(void);
 
+void tftf_initialise_timer_secondary_core(void);
+
 /*
  * Requests the timer framework to send an interrupt after milli_secs.
  * The interrupt is sent to the calling core of this api. The actual