Add missing features to the Tegra GIC driver

Add missing features to the Tegra GIC driver

In order to handle secure/non-secure interrupts, overload the plat_ic_*
functions and copy GIC helper functions from arm_gic.c. Use arm_gic.c
as the reference to add Tegra's GIC helper functions.

Now that Tegra has its own GIC implementation, we have no use for
plat_gic.c and arm_gic.c files.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>