x86/platform/uv/BAU: Replace hard-coded values with MMR definitions

Replaces hard-coded node ID shift for the descriptor base MMR to fix
initialization on UV4A while maintaining support for previous architectures.

Signed-off-by: Andrew Banman <abanman@hpe.com>
Acked-by: Mike Travis <mike.travis@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dimitri Sivanich <sivanich@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russ Anderson <rja@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1515440592-44060-1-git-send-email-abanman@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>