Revert "ARM: mutex: use generic atomic_dec-based implementation for ARMv6+"

This reverts commit bbe98c10312d9e3ef33169873c947a814cde9f6a.
Revert "ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+"

This reverts commit 58cab3be4dd24c3532666898d1d77efcb7fb9a27.