ACPI: property: Support strings in Apple _DSM props

The Wi-Fi module in x86 Apple machines has a "module-instance" device
property that specifies the platform type and is used for firmware
selection. Its value is a string, so add support for string values in
acpi_extract_apple_properties().

Reviewed-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>