board: topic-miamiplus: Run IO PLL at 1000 MHz

The miamiplus can use GEM0 through MIO pins, which requires a 125 MHz TX
clock to be generated. With the IO PLL at 1200 MHz this isn't possible, so
change it to run at 1000 and adjust the divisors accordingly. Also set the
GEM0 clock source to MIO instead of EMIO.

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