clk: sifive: Add a driver for the SiFive FU740 PRCI IP block

Add driver code for the SiFive FU740 PRCI IP block. This IP block
handles reset and clock control for the SiFive FU740 device and
implements SoC-level clock tree controls and dividers.

The link of unmatched as follow, and the U740-C000 manual would
be present in the same page as soon.
https://www.sifive.com/boards/hifive-unmatched



This driver contains bug fixes and contributions from
Henry Styles <hes@sifive.com>
Erik Danie <erik.danie@sifive.com>
Pragnesh Patel <pragnesh.patel@sifive.com>

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Pragnesh Patel <Pragnesh.patel@sifive.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Henry Styles <hes@sifive.com>
Cc: Erik Danie <erik.danie@sifive.com>
Cc: Pragnesh Patel <pragnesh.patel@sifive.com>
Link: https://lore.kernel.org/r/20201209094916.17383-4-zong.li@sifive.com


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