clk: microchip: mpfs: convert periph_clk to clk_gate

With the reset code moved to the recently added reset controller, there
is no need for custom ops any longer. Remove the custom ops and the
custom struct by converting to a clk_gate.

Reviewed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220909123123.2699583-14-conor.dooley@microchip.com