dt-bindings: clk: mpfs: add defines for two new clocks

The RTC reference and MSSPLL were previously not documented or defined,
as they were unused. Add their defines to the PolarFire SoC header.

Fixes: 2145bb68

 ("dt-bindings: clk: microchip: Add Microchip PolarFire host binding")
Reviewed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220413075835.3354193-6-conor.dooley@microchip.com
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>