arm: enable allocate-on-read for LPAE's DCACHE_WRITEBACK/_WRITETHROUGH

The LPAE versions of DCACHE_WRITEBACK and DCACHE_WRITETHROUGH are currently
defined as no-allocate for both reads and writes, which deviates from the
non-LPAE definition, and mostly defeats the purpose of enabling the caches
in the first place.

So align LPAE with !LPAE, and enable allocate-on-read for both. And while
at it, add some clarification about the meaning of the chosen values.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>