periph/uart: add support for enhanced serial settings

As suggested in PR#5899 add a routine uart_mode() that will
setup data bits, stop bits and parity at runtime.

uart.h provides a set of enums defining these settings and each
platform will override them to specify values corresponding to
its configuration registers.

The idea behind the enums is to specify default settings i.e. 8N1
through the 0 value item. Invoking uart_mode(uart, 0, 0, 0) will
setup 8N1 mode.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>