Only show the U64 clock driver on RISC-V

This will probably only be on a RISC-V system, and it doesn't build on
32-bit systems without 64-bit divides which is a headache.

Signed-off-by: Palmer Dabbelt <[email protected]>

Author: palmer-dabbelt

drivers/clk/sifive/Kconfig

@@ -1,4 +1,5 @@
 config CLK_U54_PRCI
 	bool "PRCI driver for U54 SoCs"
+	depends on RISCV
 	---help---
 	  Supports Power Reset Clock interface found in U540 SoCs