tty: kconfig: remove the superfluous dependency on PPC_OF

In the current kernel, the CONFIG_PPC_OF is always 'y' for the ppc
arch. So we don't need to check it with other ppc specific options.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>