clk: microchip: mpfs: simplify control reg access

The control reg addresses are known when the clocks are registered, so
we can, instead of assigning a base pointer to the structs, assign the
control reg addresses directly. Accordingly, remove the interim
variables used during reads/writes to those registers.

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/20220909123123.2699583-11-conor.dooley@microchip.com