asm-generic: make simd.h a mandatory include/asm header

The generic aegis128 software crypto driver recently gained support
for using SIMD intrinsics to increase performance, for which it
uncondionally #include's the <asm/simd.h> header. Unfortunately,
this header does not exist on many architectures, resulting in
build failures.

Since asm-generic already has a version of simd.h, let's make it
a mandatory header so that it gets instantiated on all architectures
that don't provide their own version.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>