[ARM] pxa: introduce processor specific pxa27x_assert_ac97reset()

This is really pxa27x specific and should be kept in pxa27x.c. With this
newly introduced function, the original set_resetgpio_mode() is deprecated.

Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>