serial: mpc52xx: remove reference to .set_wake()

This callback is gone and not coming back, so will not be
supported later.

Cc: Gerhard Sittig <gsi@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>