boards: arm: Clean up lpcxpresso55s16 board.cmake
boards: arm: Clean up lpcxpresso55s16 board.cmake

Cleans up the lpcxpresso55s16 board.cmake to make it consistent with
other lpc boards. jlink.board.cmake sets the default runner to jlink if
not already set.

Signed-off-by: Maureen Helm <87a696db5547fa477bda9f9e0f00013c839c14d3@nxp.com>

zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr