clk: microchip: mpfs: add RTCREF clock control

The reference clock used by the PolarFire SoC's onboard rtc was missing
from the clock driver. Add this clock at the "config" clock level, with
the external reference clock as its parent.

Fixes: 635e5e73

 ("clk: microchip: Add driver for Microchip PolarFire SoC")
Reviewed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220413075835.3354193-9-conor.dooley@microchip.com
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>