Instead of blacklisting which architectures support MMUs (mandatorily
or optionally), introduce two Kconfig options that are selected by each
architecture in each case.

This simplifies the logic in BR2_USE_MMU.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>