io-pgtable-fast: Remove av8l_fast_cfg

Fastmap uses the same page table format as arm_lpae_s1_cfg,
so they can share the same data structure format. Additionally,
arm-smmu.c *assumes* that struct arm_lpae_s1_cfg is in use.

This has not caused any issues so far because the formats of
av8l_fast_cfg and arm_lpae_s1_cfg are sufficiently similar.

Change-Id: I93dc42f82d988318af35125df3a5fa0739ebae6a
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>