clk: sifive: fu540-prci: Declare static const variable 'prci_clk_fu540' where it's used

Fixes the following W=1 kernel build warning(s):

 drivers/clk/sifive/fu540-prci.h:16:35: warning: ‘prci_clk_fu540’ defined but not used [-Wunused-const-variable=]
 drivers/clk/sifive/fu540-prci.h:16:35: warning: ‘prci_clk_fu540’ defined but not used [-Wunused-const-variable=]

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Pragnesh Patel <Pragnesh.patel@sifive.com>
Cc: Zong Li <zong.li@sifive.com>
Cc: linux-clk@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210120093040.1719407-7-lee.jones@linaro.org


Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>