Also in this commit:
1. `kinetis_periph_enable()` for enabling clocks on various MCU modules.
2. Minimal GPIO driver with the `kinetis_gpio_config()` function.

FIFOs are not implemented in this driver (they can be enables via the
PFIFO, TXWATER and RXWATER registers), because on K60 only UART0
has a more than single-dataword FIFO, and UART0 is not used on the
TWR-K60N512 board.

We use the UART3 on the TWR-K60N512 board which is connected to the DB-9
port on the TWR-SER board in the TWR-K60N512-KIT board set.