arm64: atomics: move ll/sc atomics into separate header file

In preparation for the Large System Extension (LSE) atomic instructions
introduced by ARM v8.1, move the current exclusive load/store (LL/SC)
atomics into their own header file.

Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>