ASoC: adau1701: allow configuration of PLL mode pins

The ADAU1701 has 2 hardware pins to configure the PLL mode in accordance
to the MCLK-to-LRCLK ratio. These pins have to be stable before the chip
is released from reset, and a full reset cycle, including a new firmware
download is needed whenever they change.

This patch adds GPIO properties to the DT bindings of the Codec, and
implements makes the set_sysclk memorize the configured sysclk.

Because the run-time parameters are unknown at probe time, the first
firmware download is postponed to the first hw_params call, when the
driver can determine the mclk/lrclk divider. Subsequent downloads
are only issued when the divider configuration changes.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>