drivers: net : cpsw: Update Kconfig for CPSW

CPSW is present in AM33xx, AM43xx, DRA7xx.
Updating the Kconfig to depend on ARCH_OMAP2PLUS instead of listing
all SoC's.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>