rockchip: rk3399-puma: set gpio4cd iodomain to 1.8V

The PCIe reset signal is connected to GPIO4_C6 on the Puma
module. This pin is supplied by 1.8V, but the default iodomain
setting is 3.0V and in this situation the pin is unable to go
high.

Linux assumes that this signal works in early boot
as PCIe is probed before loading the iodomain driver.

Make PCIe work in Linux by setting the gpio4cd iodomain to 1.8V.

Signed-off-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>