[platform][riscv] add code to read the size of memory from the FDT

Tweak the novm allocator to let us more easily add a variable sized
arena at boot.

Also added code to trap secondary cpus and reenable the use of WFI
instruction.

arch/riscv/arch.c

@@ -40,7 +40,9 @@ void arch_init(void) {
 void arch_idle(void) {
     // disabled for now, QEMU seems to have some trouble emulating wfi properly
     // also have trouble breaking into sifive-e board with openocd when wfi
-//    __asm__ volatile("wfi");
+    // NOTE: reenabling for now, will need to re-test on sifive board to see if this
+    // problem went away.
+    __asm__ volatile("wfi");
 }
 
 void arch_chain_load(void *entry, ulong arg0, ulong arg1, ulong arg2, ulong arg3) {