ARM: fiq_debugger: add process context reboot command

kernel_restart cannot be called from interrupt context.  Add support for
commands called from a work function, and implement the "reboot" command
there.  Also rename the existing irq-mode command to "reset" and change
it to use machine_restart instead of kernel_restart.

Change-Id: I3c423147c01db03d89e95a5b99096ca89462079f
Signed-off-by: Colin Cross <ccross@android.com>