ipq806x: implement GPIO API

Add implementation of the GPIO API defined in src/include/gpiolib.h.
Also, clean up the GPIO driver, make it use pointers instead of
integers for register address.

This requires a touch in the SPI driver, where the CS GPIO is toggled
and in the board function where it enables USB interface.

BUG=chrome-os-partner:30489
TEST=tested with the following patches, observed proto0 properly read
     the board ID.

Original-Change-Id: I0962947c6bb32a854ca300752d259a48e9e7b4eb
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/210115
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit e951f735001509d135cc61530ed0eecb5fc31a85)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: I8a612dce000931835054086c1b02ebfc43dc57d2
Reviewed-on: http://review.coreboot.org/8718
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/google/storm/mainboard.c

@@ -40,6 +40,7 @@ static void setup_usb(void)
 #if !CONFIG_BOARD_VARIANT_AP148
 	gpio_tlmm_config_set(USB_ENABLE_GPIO, FUNC_SEL_GPIO,
 			     GPIO_PULL_UP, GPIO_10MA, GPIO_ENABLE);
+	gpio_set_out_value(USB_ENABLE_GPIO, 1);
 #endif
 	usb_clock_config();