Currently NuttX on Arm64 supports Generic Interrupt Controller (GIC) Versions 3 and 4: [`arm64_gicv3.c`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm64/src/common/arm64_gicv3.c), [`arm64_gic.h`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm64/src/common/arm64_gic.h). This PR adds support for GIC Version 2, which is needed by [Pine64 PinePhone](https://lupyuen.github.io/articles/interrupt) based on Allwinner A64 SoC.
This 64-bit implementation of GIC v2 is mostly identical to the existing GIC v2 for 32-bit Armv7-A ([`armv7-a/arm_gicv2.c`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm/src/armv7-a/arm_gicv2.c), [`armv7-a/gic.h`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm/src/armv7-a/gic.h)), with minor modifications to support 64-bit Registers (Interrupt Context).
- `arch/arm64/Kconfig`: Under "ARM64 Options", we added an integer option `ARM_GIC_VERSION` ("GIC version") that selects the GIC Version. Valid values are 2, 3 and 4, default is 3.
- `arch/arm64/src/common/arm64_gicv2.c`: Implements 64-bit GIC v2 based on 32-bit [`armv7-a/arm_gicv2.c`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm/src/armv7-a/arm_gicv2.c) and [`armv7-a/gic.h`](https://github.com/apache/incubator-nuttx/blob/master/arch/arm/src/armv7-a/gic.h), modified to support 64-bit Registers (Interrupt Context).
Function and Macro Names have not been changed, for easier cross-referencing between the 32-bit and 64-bit implementations of GIC v2.
- `arch/arm64/src/common/arm64_gicv3.c`: Added Conditional Compilation for GIC v3. This file will not be compiled if `ARM_GIC_VERSION` is 2.
- `arch/arm64/src/common/arm64_gic.h`: Added the Version Identifier for GIC v2. At startup we read the GIC Version from hardware and verify that it matches `ARM_GIC_VERSION`.
- `arch/arm64/include/qemu/chip.h`: Added the QEMU Base Addresses for GIC v2.
- `arch/arm64/src/common/Make.defs`: Added the source file that implements GIC v2.
- `boards/arm64/qemu/qemu-armv8a/README.txt`: Added the documentation for testing GIC v2 with QEMU.
- `boards/arm64/qemu/qemu-armv8a/configs/nsh_gicv2/defconfig`: Added the Board Configuration `qemu-armv8a:nsh_gicv2` for testing GIC v2 with QEMU. Identical to `qemu-armv8a:nsh`, except that `ARM_GIC_VERSION` is 2.
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