Use SPDX license identifiers ...

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

Prepare platforms to use refactored ARM GIC drivers ...

This patch adds platform helpers for the new GICv2 and GICv3 drivers in
plat_gicv2.c and plat_gicv3.c. The platforms can include the appropriate
file in their build according to the GIC driver to be used. The existing
plat_gic.c is only meant for the legacy GIC driver.

In the case of ARM platforms, the major changes are as follows:

1. The crash reporting helper macro `arm_print_gic_regs` that prints the GIC CPU
   interface register values has been modified to detect the type of CPU
   interface being used (System register or memory mappped interface) before
   using the right interface to print the registers.

2. The power management helper function that is called after a core is powered
   up has been further refactored. This is to highlight that the per-cpu
   distributor interface should be initialised only when the core was originally
   powered down using the CPU_OFF PSCI API and not when the CPU_SUSPEND PSCI API
   was used.

3. In the case of CSS platforms, the system power domain restore helper
   `arm_system_pwr_domain_resume()` is now only invoked in the `suspend_finish`
   handler as the system power domain is always expected to be initialized when
   the `on_finish` handler is invoked.

Change-Id: I7fc27d61fc6c2a60cea2436b676c5737d0257df6