kernel: no chacha-mips.ko on mips32 r1 targets

CHACHA_MIPS depends on CPU_MIPS32_R2. Therefore,
kmod-crypto-lib-chacha20 should not contain chacha-mips.ko on MIPS32 R1
targets. Enforce that in the target-specific definition.

Fixes bcm47xx, bcm63xx, lantiq/ase, ath25 builds.

Fixes: 06351f1b ("kernel: migrate wireguard into the kernel tree")
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>