locking/atomic: h8300: use asm-generic exclusively

As h8300's implementation of the atomics isn't using any arch-specific
functionality, and its implementation of cmpxchg only uses assembly to
non-atomically swap two elements in memory, we may as well use the
asm-generic atomic.h and cmpxchg.h, and avoid the duplicate code.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210525140232.53872-4-mark.rutland@arm.com