ARM: move AES typedefs and function prototypes to separate header

Put the struct definitions for AES keys and the asm function prototypes in a
separate header and export the asm functions from the module.
This allows other drivers to use them directly.

Change-Id: I4c5c4a57b917cb2dab1dd1b2c05c958619ab23a4
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>