pinctrl/gpiolib: Mark gpiochips as "used" for LTO

GCC's Link Time Optimisation feature can cause the link-to-register
feature of gpiolib to fail - without an explicit reference to the data
it is assumed to be unused, and dropped. Add the "used" attribute to
prevent this.

See: #75

Signed-off-by: Phil Elwell <[email protected]>

pinctrl/gpiochip.h

@@ -4,7 +4,7 @@
 #include "gpiolib.h"
 
 #define DECLARE_GPIO_CHIP(name, compatible, iface, size, data) \
-    GPIO_CHIP_T name ## _chip __attribute__ ((section ("gpiochips"))) = \
+    GPIO_CHIP_T name ## _chip __attribute__ ((section ("gpiochips"))) __attribute__ ((used)) = \
     { #name, compatible, iface, size, data }
 
 typedef struct GPIO_CHIP_INTERFACE_ GPIO_CHIP_INTERFACE_T;