taskprocessor.c: Add CLI commands to reset taskprocessor stats.

Added two new CLI commands to reset stats for taskprocessors. You can
reset stats for a single, specific taskprocessor ('core reset
taskprocessor <taskprocessor>'), or you can reset all taskprocessors
('core reset taskprocessors'). These commands will reset the counter for
the number of tasks processed as well as the max queue size.

Change-Id: Iaf17fc4ae29396ab0c6ac92408fc7bdc2f12362d