tile: use __ro_after_init instead of tile-specific __write_once

The semantics of the old tile __write_once are the same as the
newer generic __ro_after_init, so rename them all and get rid
of the tile-specific version.

This does not enable actual support for __ro_after_init,
which had been dropped from the tile architecture before the
initial upstreaming was done, since we had at that time switched
to using 16MB huge pages to map the kernel.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>