dt-bindings: clock: sifive: add FU540-C000 PRCI clock constants

Add preprocessor macros for the important PRCI output clocks
that are needed by both the FU540 PRCI driver and DT data.
Details are available in the FU540 manual in Chapter 7 of

    https://static.dev.sifive.com/FU540-C000-v1.0.pdf

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>