Remove re-initialisation of system timers after warm boot for FVP

This patch removes the reinitialisation of memory mapped system timer
registers after a warm boot for the FVP. This is done in FVP because the
system timers are in the 'Always ON' power domain and we could potentially
overwrite the system timer setup done by the normal world after a warm
boot.

With this patch, the system timer registers CNTACR(x) and CNTNSAR are
programmed only during the cold boot.

Fixes ARM-software/tf-issues#169

Change-Id: Ie982eb03d1836b15ef3cf1568de2ea68a08b443e