misc: at25: Make use of device property API

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>