RISCV: Support cpu hotplug.

This patch enable support for cpu hotplug in RISC-V.

In absensece of generic cpu stop functions, WFI is used
to put the cpu in low power state during offline. An IPI
is sent to bring it out of WFI during online operation.

Tested both on QEMU and HighFive Unleashed board with
4 cpus. Test result follows.

$ echo 0 > /sys/devices/system/cpu/cpu2/online
[   31.828562] CPU2: shutdown
$ cat /proc/cpuinfo
hart    : 1
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

hart    : 3
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

hart    : 4
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

$ echo 0 > /sys/devices/system/cpu/cpu4/online
[   52.968495] CPU4: shutdown
$ cat /proc/cpuinfo
hart    : 1
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

hart    : 3
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

$ echo 1 > /sys/devices/system/cpu/cpu4/online
[   64.298250] CPU4: online
$ cat /proc/cpuinfo
hart    : 1
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

hart    : 3
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

hart    : 4
isa     : rv64imafdc
mmu     : sv39
uarch   : sifive,rocket0

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>