Add EPs for Atmel SAM L21 QFM packages

Fix for #3224 (closed).

All Atmel SAM L21 QFN (suffix -MUT ) parts were missing the EP GND connection ,required by the DS:

This applies to the three families:

• SAM L21J - 64 pin + EP QFN
• SAM L21G - 48 pin + EP QFN
• SAM L21E - 32 pin + EP QFN.

It does not apply to the QFP -AUT parts as they don't have EPs.

All EP GNDs are stacked with the other GND pins, as in the SAME libraray.

In a separate commit, also update the DS URLs in all L21 parts to the current one: https://ww1.microchip.com/downloads/aemDocuments/documents/MCU32/ProductDocuments/DataSheets/SAM-L21-Family-Data-Sheet-DS60001477.pdf.