aarch64: Update aarch64-sys-regs.def header

Given the shared use of the aarch64-sys-regs.def file across Binutils
and GCC, add instructions for keeping the file synchronized across the
two codebases.

Namely, it should be made clear that all changes are first to be made
in Binutils and the updated file copied across to GCC.

opcodes/ChangeLog

	* opcodes/aarch64-sys-regs.def: Update file-description header
	comment.