arm64: Move documentation of dcache_by_line_op

arm64: Move documentation of dcache_by_line_op

The comment describing the macro dcache_by_line_op is placed
right before the previous macro of the one it describes, which is
a bit confusing. Move it to the macro it describes (dcache_by_line_op).

No functional change intended.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210524083001.2586635-9-tabba@google.com
Signed-off-by: Will Deacon <will@kernel.org>