boards: nrf52: Specify nRF52832 IC in default config

In preparation for the new Nordic MDK and nRF52840 IC support this patch
adds the specific nRF52832 IC model in the pca10040 board default
config, so that future boards with different ICs from the same nRF52
series are supported correctly.

JIRA: ZEP-1418

Change-Id: Ic7c2076eeeb33fb729e4dbba2ff4702ab812a826
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>