topic-miamiplus: Run CPU at 800MHz for speedgrade-2

The miamiplus contains a speedgrade-2 device, which may run the CPU at 800MHz.
Change the PLL setting to 800MHz, and adapt the setpoints in the devicetree.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>