clk: sifive: Extract prci core to common base

Extract common core of prci driver to an independent file, it could
allow other chips to reuse it. Separate SoCs-dependent code 'fu540'
from prci core, then we can easily add 'fu740' later.

Almost these changes are code movement. The different is adding the
private data for each SoC use, so it needs to get match data in probe
callback function, then use the data for initialization.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Pragnesh Patel <Pragnesh.patel@sifive.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Link: https://lore.kernel.org/r/20201209094916.17383-2-zong.li@sifive.com


[sboyd@kernel.org: Include header to silence sparse]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>