cpu/nrf51: define GPIO_COUNT

GPIO_COUNT is not defined in the vendor headers, but it's always one
for this family (one GPIO port).

cpu/nrf51/include/cpu_conf.h

@@ -57,6 +57,11 @@ extern "C" {
 #define FLASHPAGE_WRITE_BLOCK_ALIGNMENT (4U)
 /** @} */
 
+/**
+ * @brief   nRF51 only has one GPIO block
+ */
+#define GPIO_COUNT              (1U)
+
 /**
  * @brief   Due to RAM restrictions, we need to limit the default GNRC packet
  *          buffer size on these CPUs