ARM: 8698/1: dma-mapping: Mark atomic_pool as __ro_after_init

atomic_pool is setup once while init stage and never changed after
that, so it is good candidate for __ro_after_init.

Since we are here mark atomic_pool_size with __init_data.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>