Fix and expand comments for the cache maintenance functions in
cacheflush.h. Adds comments to functions that weren't described
before. Explains what the functions do using Arm Architecture
Reference Manual terminology.

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/20210524083001.2586635-18-tabba@google.comSigned-off-by: Will Deacon <will@kernel.org>