net: ethernet: ti: Add dependency for TI_DAVINCI_EMAC

If TI_DAVINCI_EMAC=y and GENERIC_ALLOCATOR is not set,
below erros can be seen:
drivers/net/ethernet/ti/davinci_cpdma.o: In function `cpdma_desc_pool_destroy.isra.14':
davinci_cpdma.c:(.text+0x359): undefined reference to `gen_pool_size'
davinci_cpdma.c:(.text+0x365): undefined reference to `gen_pool_avail'
davinci_cpdma.c:(.text+0x373): undefined reference to `gen_pool_avail'
davinci_cpdma.c:(.text+0x37f): undefined reference to `gen_pool_size'
drivers/net/ethernet/ti/davinci_cpdma.o: In function `__cpdma_chan_free':
davinci_cpdma.c:(.text+0x4a2): undefined reference to `gen_pool_free_owner'
drivers/net/ethernet/ti/davinci_cpdma.o: In function `cpdma_chan_submit_si':
davinci_cpdma.c:(.text+0x66c): undefined reference to `gen_pool_alloc_algo_owner'
davinci_cpdma.c:(.text+0x805): undefined reference to `gen_pool_free_owner'
drivers/net/ethernet/ti/davinci_cpdma.o: In function `cpdma_ctlr_create':
davinci_cpdma.c:(.text+0xabd): undefined reference to `devm_gen_pool_create'
davinci_cpdma.c:(.text+0xb79): undefined reference to `gen_pool_add_owner'
drivers/net/ethernet/ti/davinci_cpdma.o: In function `cpdma_check_free_tx_desc':
davinci_cpdma.c:(.text+0x16c6): undefined reference to `gen_pool_avail'

This patch mades TI_DAVINCI_EMAC select GENERIC_ALLOCATOR.

Fixes: 99f62971 ("net: ethernet: ti: cpsw: drop TI_DAVINCI_CPDMA config option")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>