AArch64: Add atomic functions

This patch adds atomic functions for AArch64. The functions
will be revisited later for potential optimizations using
assembly or otherwise.

This patch also introduces new 64-bit atomic functions for
LP64 platforms.

Change-Id: Id2127dd01cea65025f939e955d73d27e95d8687e
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>