hwmon: Add xgene hwmon driver

This patch adds hardware temperature and power reading support for
APM X-Gene SoC using the mailbox communication interface.

Signed-off-by: Hoan Tran <hotran@apm.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>