crypto: stm32 - Use accelerated readsl/writesl

When reading or writing crypto buffers the inner loops can
be replaced with readsl and writesl which will on ARM result
in a tight assembly loop, speeding up encryption/decryption
a little bit. This optimization was in the Ux500 driver so
let's carry it over to the STM32 driver.

Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Lionel Debieve <lionel.debieve@foss.st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>