Add initial ICU driver for AVR

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5939 35acf78f-673a-0410-8e92-d51de3d6d3f4
Fix PWM regtable conflict with ICU

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5940 35acf78f-673a-0410-8e92-d51de3d6d3f4