Replacing vtbl NEON intrinsic with rev NEON intrinsic for byte_swap.

Replacing vtbl NEON intrinsic with rev NEON intrinsic for byte_swap.

Using rev16 vector intrinsic to reverse byteorder in each word
independently.

Change-Id: I071c40780baffe0bda614ec5d9dd92858f574b0d
Signed-off-by: Sirshak Das <sirshak.das@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Brian Brooks <brian.brooks@arm.com>
Reviewed-by: Yi He <yi.he@arm.com>
Verified-by: Lijian Zhang <lijian.zhang@arm.com>