nrf/board: Migrate nrf52840 target to new BLE stack.

This patch moves pca10056/nrf52840 target board to use
the new SoftDevice s140 v6.1.1 instead of the legacy v6.0.0.

ports/nrf/boards/pca10056/mpconfigboard.mk

 MCU_SERIES = m4
 MCU_VARIANT = nrf52
 MCU_SUB_VARIANT = nrf52840
-SOFTDEV_VERSION = 6.0.0
+SOFTDEV_VERSION = 6.1.1
 LD_FILES += boards/nrf52840_1M_256k.ld
 
 NRF_DEFINES += -DNRF52840_XXAA