gpio: zynq: Clarify quirk and provide helper function

The one quirk used in the zynq GPIO driver was called FOO which is not
very descriptive. Rename the quirk to IS_ZYNQ as it indicates whether
the HW is a zynq or zynqmp device to allow handling of device-specific
differences of the HW.
Also provide a helper function to test whether the HW is zynq or zynqmp.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>