clk: sifive: restrict Kconfig scope for the FU540 PRCI driver

Restrict Kconfig scope for SiFive clock and reset IP block drivers
such that they won't appear on most configurations that are unlikely
to support them.  This is based on a suggestion from Pavel Machek
<pavel@ucw.cz>.  Ideally this should be dependent on
CONFIG_ARCH_SIFIVE, but since that Kconfig directive does not yet
exist, add dependencies on RISCV or COMPILE_TEST for now.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reported-by: Pavel Machek <pavel@ucw.cz>
Cc: Michael Turquette <mturquette@baylibre.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>