ACPI: Provide acpi_dev_name accessor for struct acpi_device device name

struct acpi_device fields are only available when CONFIG_ACPI is set. We may
find use for dev_name(&adev->dev) in generic code that is build also without
CONFIG_ACPI is set but currently this requires #ifdef CONFIG_ACPI churn.

Provide here an accessor that returns dev_name of embedded struct device dev
in struct acpi_device or NULL depending on CONFIG_ACPI setting.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>