clk: sifive: Move all stuff into SoCs header files from C files

Improve PRCI driver to reduce the complexity, we remove the SoCs C files
by putting all stuff in each SoCs header files, and include these
SoCs-specific header files in core of PRCI. It can also avoid the W=1
kernel build warnings about variable defined but not used
[-Wunused-const-variable=], like commit 487dc7bb

 ("clk: sifive:
fu540-prci: Declare static const variable 'prci_clk_fu540' where it's
used") does.

Signed-off-by: Zong Li <zong.li@sifive.com>
Suggested-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/a3c7ec5c46c1d8be455d1c347db4855bb56cec53.1646388139.git.zong.li@sifive.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>