RISC-V: Replace hardcoded constants with enum values

The RISC-V device-tree code has a number of hard-coded
constants and this change moves them into header enums.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

include/hw/riscv/sifive_u.h

@@ -50,6 +50,10 @@ enum {
     SIFIVE_U_UART1_IRQ = 4
 };
 
+enum {
+    SIFIVE_U_CLOCK_FREQ = 1000000000
+};
+
 #define SIFIVE_U_PLIC_HART_CONFIG "MS"
 #define SIFIVE_U_PLIC_NUM_SOURCES 127
 #define SIFIVE_U_PLIC_NUM_PRIORITIES 7