crypto: omap-sham - make fallback size configurable

Crypto driver fallback size can now be configured from userspace. This
allows optimizing the DMA usage based on use case. Default fallback
size of 256 is still used.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>