Add arm64 configuration and device tree differences. Disable MMC_BCM2835_SDHOST and...

Add arm64 configuration and device tree differences. Disable MMC_BCM2835_SDHOST and MMC_BCM2835 since these drivers are crashing at the moment.

ARM64: Modify default config to get raspbian to boot (#1686)

1. Enable emulation of deprecated instructions.
2. Enable ARM 8.1 and 8.2 features which are not detected at runtime.
3. Switch the default governer to powersave.
4. Include the watchdog timer driver in the kernel image rather then a module.

Tested with raspbian-jessie 2016-09-23.