s390/tty3270: resize terminal when the clear key is pressed

There's no easy way to figure out whether the user has re-connected
to the z/VM session. When the user re-connected with a different geometry
to z/VM, the screen layout is broken. Allow the user to force a resizing
by pressing the Clear Key.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>