drivers/net/dsa/Kconfig: Let NET_DSA_BCM_SF2 depend on HAS_IOMEM

NET_DSA_BCM_SF2 need HAS_IOMEM, so depend on it, the related error (with
allmodconfig under um):

    CC [M]  drivers/net/dsa/bcm_sf2.o
  drivers/net/dsa/bcm_sf2.c: In function ‘bcm_sf2_sw_setup’:
  drivers/net/dsa/bcm_sf2.c:487:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
     iounmap(*base);
     ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>