AArch64: Clean up docs and document -mcpu and -march.

2014-03-13  Richard Earnshaw  <rearnsha@arm.com>
	    Jiong Wang  <Jiong.Wang@arm.com>

	* doc/c-aarch64.texi: Clean up some formatting issues.
	(AArch64 Options): Document -mcpu and -march.
	(AArch64 Extensions): New node.