riscv: dts: microchip: use clk defines for icicle kit

Update the Microchip Icicle kit device tree by replacing clock
related magic numbers with their defined counterparts.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>