add DTS gpio support for NRF51

[galak]

The NRF51 platforms / SoCs need dts support for HAS_DTS_GPIO

[Mierunski]

I'm currently working on shim for GPIO using NRFX. For that I will also add dts support for nRF51

[carlescufi]

@Mierunski thanks. Then I will assign this to you and we can close it as part of the PR. Please add Fixes #8713 to the PR or commit message.