[nrf][pca10040] fix platform incompatibilities

When the nrf52xxx platform was changed to use the Nordic nrfx
libraries the pca10040 target was broken due to differences in
how the platform handled initialization.

platform/nrf52xxx/init.c

@@ -47,5 +47,7 @@ void platform_init(void) {
 
     // Note: i2c_init will only instantiate an i2c device if proper defines
     //   are set.  See comments at top of i2c_master.c for more info.
+#if (NRFX_TWIM_ENABLED)
     i2c_init();
+#endif
 }

target/nrf-pca10040/include/target/gpioconfig.h

@@ -8,8 +8,6 @@
 #ifndef __TARGET_GPIOCONFIG_H
 #define __TARGET_GPIOCONFIG_H
 
-#include <platform/gpio.h>
-
 #define GPIO_LED1   17
 #define GPIO_LED2   18
 #define GPIO_LED3   19

target/nrf-pca10040/init.c

@@ -10,8 +10,7 @@
 #include <target.h>
 #include <lk/compiler.h>
 #include <dev/gpio.h>
-#include <platform/gpio.h>
-#include <platform/nrf52.h>
+#include <platform/init.h>
 #include <target/gpioconfig.h>
 
 void target_early_init(void) {