arm64: make ll/sc __cmpxchg_case_##name asm consistent

The ll/sc __cmpxchg_case_##name assembly mostly uses symbolic names for
operands, but in a single case uses %2 to refer to what is otherwise
known as %[v]. This makes the code more painful to read than is
necessary.

Use %[v] instead.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>