at91: Refactor Stamp9G20 and PControl G20 board file

As PControl G20 is a carrier board for the Stamp9G20 SoM, some code can
be shared. Therefore board-stamp9g20.c is refactored to allow reusing the
SoM initialization and board-pcontrol-g20.c is modified to use it.

Signed-off-by: Christian Glindkamp <christian.glindkamp@taskit.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>