Add two helper functions for configuring and enabling generic clocks:
  - gclk_enable_output: Enables output on a GCLKx pin
  - gclk_set_rate: Configures a gclk to run at a specific rate

This should eliminate any reason to go mucking about with PM registers
from board code.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>