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>