arm: mvebu: Remove unused alias from RC AC5X dts

The sar-reg0 alias was left over from an earlier iteration of the
patches adding support for this board. Remove the unused alias.

Fixes: 6cc8b5db ("arm: mvebu: Add RD-AC5X board")
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>