MIPS: Initialise MAARs

Add initialisation for Memory Accessibility Attribute Registers. Generic
code cannot know the platform-specific requirements with regards to
speculative accesses, so it simply calls a platform_maar_init function
which platforms with MAARs are expected to implement by calling the
provided write_maar_pair function & returning the number of MAAR pairs
used. A weak default implementation will simply use no MAAR pairs. Any
present but unused MAAR pairs are then marked invalid, effectively
disabling them.

The end result of this patch is that MAARs are all marked invalid, until
platforms implement the platform_maar_init function.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7331/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>