tty: let architectures override the user/kernel macros.

Give architectures that support the new termios2 the possibilty to overide the
user_termios_to_kernel_termios and kernel_termios_to_user_termios macros.  As
soon as all architectures that use the generic variant have been converted the
ifdefs can go away again.  Architectures in question are avr32, frv, powerpc
and s390.

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Mackerras <paulus@samba.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>