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_clint.h

@@ -47,4 +47,8 @@ enum {
     SIFIVE_TIME_BASE    = 0xBFF8
 };
 
+enum {
+    SIFIVE_CLINT_TIMEBASE_FREQ = 10000000
+};
+
 #endif