net: broadcom CNIC: requires MMU

The CNIC kconfig symbol selects UIO and UIO depends on MMU.
Since 'select' does not follow dependency chains, add the same MMU
dependency to CNIC.

Quietens this kconfig warning:

WARNING: unmet direct dependencies detected for UIO
  Depends on [n]: MMU [=n]
  Selected by [m]:
  - CNIC [=m] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_BROADCOM [=y] && PCI [=y] && (IPV6 [=m] || IPV6 [=m]=n)

Fixes: adfc5217 ("broadcom: Move the Broadcom drivers")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Rasesh Mody <rmody@marvell.com>
Cc: GR-Linux-NIC-Dev@marvell.com
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/20201129070843.3859-1-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>