gpio: gpio-zynq: shift zynq_gpio_init() to subsys_initcall level

gpio: gpio-zynq: shift zynq_gpio_init() to subsys_initcall level

In general situation on-SoC GPIO controller drivers should be probed
after pinctrl/pinmux controller driver, because on-SoC GPIOs utilize a
pin/pad as a resource provided and controlled by pinctrl subsystem.

  GPIO must come after pinctrl as gpios may need to mux pins....etc

Looking at Xilinx SoC series pinctrl drivers, zynq*_pinctrl_init()
functions are called at arch_initcall init levels,
so the change of initcall level for gpio-zynq driver from
postcore_initcall to subsys_initcall level is sufficient. Also note
that the most of GPIO controller drivers settled at subsys_initcall
level.

If pinctrl subsystem manages pads with GPIO functions, the change is
needed to avoid unwanted driver probe deferrals during kernel boot.

Signed-off-by: Nava kishore Manne <navam@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>