gpio: Add a way to read 3-way strapping pins

Using the internal vs. external pull resistors it is possible to get
27 different combinations from 3 strapping pins. Add an implementation
of this.

This involves updating the sandbox GPIO driver to model external and
(weaker) internal pull resistors. The get_value() method now takes account
of what is driving a pin:

   sandbox: GPIOD_EXT_DRIVEN - in which case GPIO_EXT_HIGH provides the
          value
   outside source - in which case GPIO_EXT_PULL_UP/DOWN indicates the
          external state and we work the final state using those flags and
          the internal GPIOD_PULL_UP/DOWN flags

Of course the outside source does not really exist in sandbox. We are just
modelling it for test purpose.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/asm-generic/gpio.h

@@ -497,6 +497,31 @@ int gpio_get_values_as_int(const int *gpio_list);
  */
 int dm_gpio_get_values_as_int(const struct gpio_desc *desc_list, int count);
 
+/**
+ * dm_gpio_get_values_as_int_base3() - Create a base-3 int from a list of GPIOs
+ *
+ * This uses pull-ups/pull-downs to figure out whether a GPIO line is externally
+ * pulled down, pulled up or floating. This allows three different strap values
+ * for each pin:
+ *    0 : external pull-down
+ *    1 : external pull-up
+ *    2 : floating
+ *
+ * With this it is possible to obtain more combinations from the same number of
+ * strapping pins, when compared to dm_gpio_get_values_as_int(). The external
+ * pull resistors should be made stronger that the internal SoC pull resistors,
+ * for this to work.
+ *
+ * With 2 pins, 6 combinations are possible, compared with 4
+ * With 3 pins, 27 are possible, compared with 8
+ *
+ * @desc_list: List of GPIOs to collect
+ * @count: Number of GPIOs
+ * @return resulting integer value, or -ve on error
+ */
+int dm_gpio_get_values_as_int_base3(struct gpio_desc *desc_list,
+				    int count);
+
 /**
  * gpio_claim_vector() - claim a number of GPIOs for input
  *
@@ -725,6 +750,21 @@ int dm_gpio_clrset_flags(struct gpio_desc *desc, ulong clr, ulong set);
  */
 int dm_gpio_set_dir_flags(struct gpio_desc *desc, ulong flags);
 
+/**
+ * dm_gpios_clrset_flags() - Sets flags for a set of GPIOs
+ *
+ * This clears and sets flags individually for each GPIO.
+ *
+ * @desc:	List of GPIOs to update
+ * @count:	Number of GPIOs in the list
+ * @clr:	Flags to clear (GPIOD_...), e.g. GPIOD_MASK_DIR if you are
+ *		changing the direction
+ * @set:	Flags to set (GPIOD_...)
+ * @return 0 if OK, -ve on error
+ */
+int dm_gpios_clrset_flags(struct gpio_desc *desc, int count, ulong clr,
+			  ulong set);
+
 /**
  * dm_gpio_get_flags() - Get flags
  *