dts: i2c: cleanup CONFIG_I2C_x_IRQ_PRI

The majority of cases of CONFIG_I2C_x_IRQ_PRI should be
DT_I2C_x_IRQ_PRI.  So go ahead and fix them up.  Only the i2c_nios
driver still uses Kconfig for getting priority.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>