dt-bindings: clk: add missing clk ids for microchip mpfs

When this binding header was initally upstreamed, the PLL clocking the
microprocessor subsystem (MSS) and the RTC reference clocks were
omitted. Add them now, matching the IDs used in Linux.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>