Fix references to the no-longer-existent misc/ directory in comments, README files, and documentation

In ARMV71-Xplained clock configuration, divider was set to 25 to get 25*12MHz=300MHz CPU clock.  The correct multiplier is 24 becaue the calculatin if (24+1)*12MHz.  So the board was running at 312MHz.  From Efim Monjak.

Add start/end LPRAM symbols.  We might need to use the _elpram symbol if we want to add some LPRAM to heap

SAMD21: Add build framework for DMA support.  Nothing there yet except for skeletal logic taken from SAM3/4.

SAML21: Since SERCOM5 usese a different output channel, it will also need a different GCLK generator