armv8/mmu: Clean up TCR programming

Use the inner shareable attribute for memory, which makes more sense
considering that this code is called when caches are being enabled.

Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>