arch: riscv: fix hangup of multicore boot

This patch fixes hangup of RISC-V multicore boot.
Currently boot sequence uses a riscv_cpu_wake_flag to notify wakeup
request for secondary core(s).

But initial value of riscv_cpu_wake_flag is undefined, so current
mechanism is going to hangup if riscv_cpu_wake_flag and mhartid of
secondary core have the same value.

This is an example situation of this problem:

- hart1: check riscv_cpu_wake_flag (value is 1) and end the loop
- hart1: set riscv_cpu_wake_flag to 0
- hart0: set riscv_cpu_wake_flag to 1
         hart0 expects it will be changed to 0 by hart1 but it
         has never happened

Note:
  - hart0's mhartid is 0, hart1's mhartid is 1
  - hart0 is main, hart1 is secondary in this example

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>