crypto: amlogic-gxl - default to build as module

The AmLogic GXL crypto accelerator driver is built into the kernel if
ARCH_MESON is set. However, given the single image policy of arm64, its
defconfig enables all platforms by default, and so ARCH_MESON is usually
enabled.

This means that the AmLogic driver causes the arm64 defconfig build to
pull in a huge chunk of the crypto stack as a builtin as well, which is
undesirable, so let's make the amlogic GXL driver default to 'm' instead.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>