Make use of device property API in this driver so that both DT and ACPI
based systems can use this driver.

In addition we hard-code the name of the chip to be "at25" for the
reason that there is no common mechanism to fetch name of the firmware
node. The only existing user (arch/arm/boot/dts/phy3250.dts) uses the
same name so it should continue to work.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>