acpi: respect const qualifier

This patch fixes next warning:

drivers/acpi/property.c: In function ‘acpi_free_properties’:
drivers/acpi/property.c:35:2: warning: passing argument 1 of ‘acpi_os_free’ discards ‘const’ qualifier from pointer target type [enabled by default]
  ACPI_FREE(adev->properties);

Signed-off-by: Semen Protsenko <semen.protsenko@linaro.org>
GUFI: add option to use _PRP method as ACPI property

Till this patch the only option for ACPI property was _DSM method.
This patch allows us to use _PRP method for this purpose.

For now we can't use _PRP method (for details see commit:
ACPI: GUFI: temporarily use the method "PROP" instead of "_PRP").
So here is an example of _PRP property usage:

Method (PROP, 0, NotSerialized) {
	Return (Package () {
		Package () { "reg-io-width", 0x123 },
	})
}

In the future "PROP" term will be replaced with "_PRP".

Signed-off-by: Semen Protsenko <semen.protsenko@linaro.org>