gpio: add generic single-register fixed-direction GPIO driver

Add a simple, generic, single register fixed-direction GPIO driver.
This is able to support a single register with a mixture of inputs
and outputs.

This is different from gpio-mmio and gpio-74xx-mmio:
* gpio-mmio doesn't allow a fixed direction, it assumes there is always
  a direction register.
* gpio-74xx-mmio only supports all-in or all-out setups
* gpio-74xx-mmio is DT only, this needs to support legacy too
* they don't double-read when getting the GPIO value, as required by
  some implementations that this driver supports
* we need to always do 32-bit reads, which bgpio doesn't guarantee
* the current output state may not be readable from the hardware
  register - reading may reflect input status but not output status.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/gpio-reg.h

@@ -0,0 +1,12 @@
+#ifndef GPIO_REG_H
+#define GPIO_REG_H
+
+struct device;
+
+struct gpio_chip *gpio_reg_init(struct device *dev, void __iomem *reg,
+	int base, int num, const char *label, u32 direction, u32 def_out,
+	const char *const *names);
+
+int gpio_reg_resume(struct gpio_chip *gc);
+
+#endif