aarch64: Enable Cortex-A520 CPU

This patch adds support for the Cortex-A520 CPU to gas.

No regressions on aarch64-none-elf.

gas/ChangeLog:

	* NEWS: Update docs.
	* config/tc-aarch64.c: Add Cortex-A520.
	* doc/c-aarch64.texi: Update docs.