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.pdfSigned-off-by: NPaul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>