crypto: atmel: fix bogus select

The Atmel at91 crypto driver unconditionally selects AT_HDMAC,
which results in a Kconfig warning if that driver is not enabled:

warning: (CRYPTO_DEV_ATMEL_AES) selects AT_HDMAC which has unmet direct dependencies (DMADEVICES && ARCH_AT91)

The crypto driver itself does not actually have a dependency
on a particular dma engine, other than this being the one that
is used in at91.

Removing the 'select' gets rid of the warning, but can cause
the driver to be unusable if the HDMAC is not enabled at the
same time. To work around that, this patch clarifies the runtime
dependency to be 'AT_HDMAC || AT_XDMAC', but adds an alternative
for COMPILE_TEST, which lets the driver get build on all systems.

The ARCH_AT91 dependency is implied by AT_XDMAC || AT_HDMAC now
and no longer needs to be listed separately.

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