boards: arm: Refactor the GPIO and UART dependencies

To make way for the upcoming blinky and GPIO support for nRF5x-based
boards, this change addresses the hardcoded dependencies in Kconfig
default configurations of the different boards, moving the common option
defaults to the SoC default configuration itself.

Change-Id: I8db0750311ad5a12b76237b39438376f20f6f496
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>