s390/tty3270: add support for VT100 graphics escape

Add support for ESC(B and ESC(0 to switch between character charset
and graphics charset. Used in vt100 and later terminal generations.

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>