PCI: amlogic: Fix build failure due to missing gpio header

Building the driver when GPIOLIB=n is not selected is causing the following
compilation failure:

  drivers/pci/controller/dwc/pci-meson.c: In function 'meson_pcie_assert_reset':
  drivers/pci/controller/dwc/pci-meson.c:290:2: error: implicit declaration of function 'gpiod_set_value_cansleep'; did you mean 'gpio_set_value_cansleep'? [-Werror=implicit-function-declaration]
    gpiod_set_value_cansleep(mp->reset_gpio, 0);
    ^~~~~~~~~~~~~~~~~~~~~~~~
    gpio_set_value_cansleep
  drivers/pci/controller/dwc/pci-meson.c: In function 'meson_pcie_probe':
  drivers/pci/controller/dwc/pci-meson.c:540:19: error: implicit declaration of function 'devm_gpiod_get'; did you mean 'devm_gpio_free'? [-Werror=implicit-function-declaration]
    mp->reset_gpio = devm_gpiod_get(dev, "reset", GPIOD_OUT_LOW);
		     ^~~~~~~~~~~~~~
		     devm_gpio_free
  drivers/pci/controller/dwc/pci-meson.c:540:48: error: 'GPIOD_OUT_LOW' undeclared (first use in this function); did you mean 'GPIOF_INIT_LOW'?
    mp->reset_gpio = devm_gpiod_get(dev, "reset", GPIOD_OUT_LOW);
						  ^~~~~~~~~~~~~
						  GPIOF_INIT_LOW

Add the missing linux/gpio/consumer.h header to fix it.

Fixes: 9c0ef6d3 ("PCI: amlogic: Add the Amlogic Meson PCIe controller driver")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>