These guidelines apply for both EFM32 and C8051/EFM8.
There should be a ground plane on side of the board that has sensors (usually the top side). There should be a hatched ground on the opposite side of the sensors (usually the bottom side). Traces between the sensors and pins should be as short as possible, not criss-crossed or running through stitching vias, and not run parallel to high frequency switching signals sucha s SPI or PWM signals.
For more information see application notes AN447 (8-bit) and AN0040 (32-bit). Application Notes can be found in Simplicity Studio using the [Application Notes] tile or here: www.silabs.com/32bit-appnotes, www.silabs.com/8bit-appnotes.