ASoC: tegra_wm8903: assume CONFIG_OF, remove platform data

Tegra only supports, and always enables, device tree. Remove all runtime
checks for DT support from the driver.

This allows removal of the hard-coded Harmony ASoC mapping table, since
Harmony only boots with DT now.

All board-specific configuration now comes from device tree, so there is
no need to have a platform_data structure. Rework the driver to parse the
device tree directly into struct tegra_wm8903.

Also some slight re-ordering of probe() so that the code more closely
resembles other drivers for easier comparison. Inparticular, the GPIO DT
parsing and initial programming are moved together for each GPIO.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>