crypto: arm/sha256 - avoid sha256 code on ARMv7-M

The sha256 assembly implementation can deal with all architecture levels
from ARMv4 to ARMv7-A, but not with ARMv7-M. Enabling it in an
ARMv7-M kernel results in this build failure:

arm-linux-gnueabi-ld: error: arch/arm/crypto/sha256_glue.o: Conflicting architecture profiles M/A
arm-linux-gnueabi-ld: failed to merge target specific data of file arch/arm/crypto/sha256_glue.o

This adds a Kconfig dependency to prevent the code from being disabled
for ARMv7-M.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>