drivers/net/can/Kconfig: Let CAN_AT91 depend on HAS_IOMEM

CAN_AT91 needs HAS_IOMEM, so depends on it. The related error (with
allmodconfig under um):

    CC [M]  drivers/net/can/at91_can.o
  drivers/net/can/at91_can.c: In function ‘at91_can_probe’:
  drivers/net/can/at91_can.c:1329:2: error: implicit declaration of function ‘ioremap_nocache’ [-Werror=implicit-function-declaration]
  addr = ioremap_nocache(res->start, resource_size(res));
    ^
  drivers/net/can/at91_can.c:1329:7: warning: assignment makes pointer from integer without a cast [enabled by default]
    addr = ioremap_nocache(res->start, resource_size(res));
         ^
  drivers/net/can/at91_can.c:1384:2: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
    iounmap(addr);
    ^

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