NFC: nxp-nci: Make firmware GPIO pin optional

There are other NXP NCI compatible NFC controllers such as the PN7150
that use an integrated firmware and therefore do not have a GPIO to
select firmware downloading mode. To support this kind of controller,
let's make the firmware GPIO optional.

Signed-off-by: Frieder Schrempf <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/nfc/nxp-nci.txt

···
 - reg: address on the bus
 - interrupts: GPIO interrupt to which the chip is connected
 - enable-gpios: Output GPIO pin used for enabling/disabling the chip
-- firmware-gpios: Output GPIO pin used to enter firmware download mode
 
 Optional SoC Specific Properties:
 - pinctrl-names: Contains only one value - "default".
 - pintctrl-0: Specifies the pin control groups used for this controller.
+- firmware-gpios: Output GPIO pin used to enter firmware download mode
 
 Example (for ARM-based BeagleBone with NPC100 NFC controller on I2C2):
 

drivers/nfc/nxp-nci/i2c.c

···
 		return PTR_ERR(phy->gpiod_en);
 	}
 
-	phy->gpiod_fw = devm_gpiod_get(dev, "firmware", GPIOD_OUT_LOW);
+	phy->gpiod_fw = devm_gpiod_get_optional(dev, "firmware", GPIOD_OUT_LOW);
 	if (IS_ERR(phy->gpiod_fw)) {
 		nfc_err(dev, "Failed to get FW gpio\n");
 		return PTR_ERR(phy->gpiod_fw);