drivers: Add basic GPIO and UART support for nRF52

Add basic support for the Nordic Semiconductor's
nRF52832 (nRF52 series) GPIO and UART.

JIRA: ZEP-377
Change-Id: Ie3613956bae57829e26cb1cc20d3f740b5371f65
Signed-off-by: Glenn Ruben Bakke <glenn.ruben.bakke@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Håkon Alseth <haakon.alseth@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>