cpu/kinetis_common: initial import for kinetis_common

  add peripheral drivers for Freescale Kinetis MCUs:
    adc driver
    cpuid driver
    gpio driver
    hwtimer_arch driver (hwtimer used Low Power Timer)
    i2c driver (master mode only)
    mcg driver
    pwm driver
    random_rnga driver
    random_rngb driver
    rtc driver
    spi driver
    timer driver (timer used Periodic Interrupt Timer)
    uart driver
  add doc.txt (configuration examples)

  random_rnga: Update RNGA driver in preparation for RNGB driver.
  random_rngb: Add RNGB driver.
  spi: refactor SPI to work for multiple CTARS, add spi_acquire, spi_release
  gpio: Add gpio_irq_enable, gpio_irq_disable. Refactor GPIO.
  gpio: Add gpio_irq_enable, gpio_irq_disable.
  gpio: Refactor ISR functions to work with all GPIOs (0-31) and all ports (PORTA-PORTH)
  adc: Refactor ADC, add calibration and scaling.
    Added integer scaling of results in adc_map.
    Handle precision setting in adc_init.
    Set ADC clock divider depending on module clock.
    Add ADC_1 as a possible device.
    Add ADC calibration procedure according to K60 ref manual.
    Handle ADC pins which are not part of the pin function mux.
Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>