RT74765. twr-k60n512: serial driver implementation
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.
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