Add driver code for the SiFive FU540 PRCI IP block.  This IP block
handles reset and clock control for the SiFive FU540 device and
implements SoC-level clock tree controls and dividers.

Based on code written by Wesley Terpstra <wesley@sifive.com>:
https://github.com/riscv/riscv-linux/commit/999529edf517ed75b56659d456d221b2ee56bb60

Boot and PLL rate change were tested on a SiFive HiFive Unleashed
board.

This version includes several changes requested by Stephen Boyd
<sboyd@kernel.org>.
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Wesley W. Terpstra <wesley@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Megan Wachs <megan@sifive.com>
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-clk@vger.kernel.org
[sboyd@kernel.org: Fix some const and ARRAY_SIZE() issues, make makefile
only descend if CLK_SIFIVE=y]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>