zephyr: add nrf52840_pca10056 target

This board, called the NRF52840-PDK in marketing materials, is a
preview development board for the nRF52840 SoC.

Flash layout (4 KiB sectors):

        User    Start   End     Size (KiB) Size (B)
        ------- ------- ------- ---------- --------
        mcuboot 0x00000 0x07FFF         32  0x08000
        Image 0 0x08000 0x73FFF        432  0x6C000
        Image 1 0x74000 0xDFFFF        432  0x6C000
        Scratch 0xE0000 0xFCFFF        116  0x1D000

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/zephyr/targets/nrf52840_pca10056.h b/boot/zephyr/targets/nrf52840_pca10056.h
new file mode 100644
index 0000000..f56c287
--- /dev/null
+++ b/boot/zephyr/targets/nrf52840_pca10056.h
@@ -0,0 +1,14 @@
+/*
+ *  Copyright (C) 2017, Linaro Ltd
+ *  SPDX-License-Identifier: Apache-2.0
+ */
+
+#define FLASH_DRIVER_NAME		CONFIG_SOC_FLASH_NRF5_DEV_NAME
+#define FLASH_ALIGN			4
+#define FLASH_AREA_IMAGE_0_OFFSET	0x08000
+#define FLASH_AREA_IMAGE_0_SIZE		0x6C000
+#define FLASH_AREA_IMAGE_1_OFFSET	0x74000
+#define FLASH_AREA_IMAGE_1_SIZE		0x6C000
+#define FLASH_AREA_IMAGE_SCRATCH_OFFSET	0xE0000
+#define FLASH_AREA_IMAGE_SCRATCH_SIZE	0x1D000
+/* Flash sector size is provided by SoC include */