ARM: fiq_debugger: add support for kgdb

Adds polling tty ops to the fiq debugger console tty, which allows
kgdb to run against an fiq debugger console.

Add a check in do_sysrq to prevent enabling kgdb from the fiq
debugger unless a flag (writable only by root) has been set. This
should make it safe to enable KGDB on a production device.

Also add a shortcut to enable the console and kgdb together, to
allow kgdb to be enabled when the shell on the console is not
responding.

Change-Id: Ifc65239ca96c9887431a6a36b9b44a539002f544
Signed-off-by: Colin Cross <ccross@android.com>