soc/intel/apollolake: Consolidate ISH enabling

Since the Integrated Sensor Hub can be disabled through devicetree.cb
as a PCI device, there is no need for a separate register variable.
Remove handling the register and update mainboards' devicetrees. Also
keep ISH disabled on both Reef and Amenia.

Change-Id: I90dbf57b353ae1b80295ecf39877b10ed21de146
Signed-off-by: Andrey Petrov <andrey.petrov@intel.com>
Reviewed-on: https://review.coreboot.org/15710
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/apollolake/chip.h

@@ -93,9 +93,6 @@ struct soc_intel_apollolake_config {
 	/* Configure serial IRQ (SERIRQ) line. */
 	enum serirq_mode serirq_mode;
 
-	/* Integrated Sensor Hub */
-	uint8_t integrated_sensor_hub_enable;
-
 	/* I2C bus configuration */
 	struct apollolake_i2c_config i2c[APOLLOLAKE_I2C_DEV_MAX];