net: mvneta: select GENERIC_ALLOCATOR

We previously relied on GENERIC_ALLOCATOR to be selected by CONFIG_ARM,
but now we can compile-test the driver on other architectures that
don't select it:

drivers/net/built-in.o: In function `mvneta_bm_remove':
mvneta_bm.c:(.text+0x4ee35): undefined reference to `gen_pool_free'

This adds an explicit select for the part of the driver that has
the dependency.

Fixes: a0627f77

 ("net: marvell: Allow drivers to be built with COMPILE_TEST")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>