clk: microchip: mpfs: convert MSSPLL outputs to clk_divider

After splitting the MSSPLL in two, the PLL outputs have become
open-coded versions of clk_divider. Drop the custom clk ops structs, and
instead use the generic clk_divider_ops.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>