nyan: Ignore the dev mode GPIO.

This GPIO is still wired up and can be manipulated from servo, but using it in
the coreboot tables causes a couple problems. First, people may not realize
that the little switch on their servo matters and may be confused when their
machine isn't doing what they told it to. Second, dev mode is considered
selected when the GPIO is high. To simplify matching voltages and to ensure
the GPIO doesn't float around when no servo is attached, it has a pull up
configured for it in the SOC. That means that when no servo is attached, the
pin will be pulled high, effectively forcing the machine into developer mode.

With might be able to reverse the polarity of the GPIO through config files
for servo which could deal with the second problem, but we would still have
the first problem.

To simplify things and to avoid unforseen problems related to this GPIO, I
think it's best to just ignore its value like other chromebooks do and to rely
on the soft dev mode setting stored elsewhere.

BUG=None
TEST=Built and booted on nyan. Entered and exited dev mode using the keyboard
and hacking get_recovery_mode_switch to return yes or no at the appropriate
times.
BRANCH=None

Change-Id: I80c5e2d290ce9f1c3d60ef45c9132ddfd8ce7680
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/174837
Reviewed-by: Ronald Minnich <rminnich@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>

src/mainboard/google/nyan/chromeos.c

@@ -81,7 +81,7 @@ void fill_lb_gpios(struct lb_gpios *gpios)
 
 int get_developer_mode_switch(void)
 {
-	return gpio_get_in_value(GPIO(Q6));
+	return 0;
 }
 
 int get_recovery_mode_switch(void)