arm: gic_v2: Skip gic_init_secure when cpu is not in el3

This would prevent configuring non-secure regs in case gic security
extensions are not emulated in Qemu.

Signed-off-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>