Tegra: Enable irq as wake-up event for cpu_standby

Tegra: Enable irq as wake-up event for cpu_standby

As per ARM ARM D1.17.2, any physical IRQ interrupt received by the PE
will be treated as a wake-up event, if SCR_EL3.IRQ is set to '1',
irrespective of the value of the PSTATE.I bit value.

This patch programs the SCR_EL3.IRQ bit before entering CPU standby
state, to allow an IRQ to wake the PE. On waking up, the previous
SCR_EL3 value is restored.

Change-Id: Ie81cf3a7668f5ac35f4bf2ecc461b91b9b60650c
Signed-off-by: Vignesh Radhakrishnan <vigneshr@nvidia.com>