arm64: hw_breakpoint: Fix warnings about implicit fallthrough

Now that -Wimplicit-fallthrough is passed to GCC by default, the kernel
build has suddenly got noisy. Annotate the two fall-through cases in our
hw_breakpoint implementation, since they are both intentional.

Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>