lpm-levels: Do not disable non-sec interrupts in suspend

When the system suspend is happening, last core disables the non-sec
interrupts at QGIC by setting the GRPEN1_EL1_NS to ZERO. This prevents
the core from detecting  any non-sec interrupts and that would result
in the system not waking up from any of interrupts. Do not modify
GRPEN1_EL1_NS register when the system is going into suspend.

Change-Id: I7d6c5047fb4743df187fe49fba18b64db3179bc9
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>