clk/zynq/pll: Fix documentation for PLL register function

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
clk/zynq/pll: Use #defines for fbdiv min/max values

Use more descriptive #defines for the minimum and maximum PLL
feedback divider.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>