Reset CNTVOFF_EL2 register before exit into EL1 on warm boot

This patch resets the value of CNTVOFF_EL2 before exit to EL1 on
warm boot. This needs to be done if only the Trusted Firmware exits
to EL1 instead of EL2, otherwise the hypervisor would be responsible
for this.

Fixes ARM-software/tf-issues#240

Change-Id: I79d54831356cf3215bcf1f251c373bd8f89db0e0