sifive_prci: Read and write PRCI registers

sifive_prci: Read and write PRCI registers

Writes to the SiFive PRCI registers are preserved while leaving the
ready bits set for the HFX/HFR oscillators and the lock bit set for the
PLL.

Signed-off-by: Nathaniel Graff <nathaniel.graff@sifive.com>
Reviewed-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>