boards/arm/efm32gg_stk3701a: correct NVS example storage partition size

Set the size of the storage partition in flash used by the NVS example
to 3 times the erase-block-size for this SoC family. In this case a
total of 12KB (as 3 erase-block-sizes is the minimum).

Signed-off-by: Oane Kingma <o.kingma@interay.com>