replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

Add seL4_UserTop and move kernelBase to the arch level

- seL4_UserTop is a new constant which represents the top of virtual
memory available to user level
- this commit also rationalises several constants (USER_TOP, kernelBase)
and moves them to the arch level, such that ports only need to define
seL4_UserTop.

Added 32-bit support for the zynqmp.

The Zynq UltraScale+ MPSoC (PLAT zynqmp) is a Multi-Processor SOC
made by Xilinx that has a quad-core Cortex-A53, a dual-core Cortex-R5
and an FPGA.

This adds 32-bit, single-core support on the the Cortex-A53 cluster.