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