Add initial PWM driver for AVR

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5924 35acf78f-673a-0410-8e92-d51de3d6d3f4
Add HAL test for AVR PWM driver

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5925 35acf78f-673a-0410-8e92-d51de3d6d3f4