drivers: serial: lpc11u6x: allow to configure data polarity

This patch adds support for the rx-invert and tx-invert device-tree
properties to the uart_lpc11u6x driver for USARTs 1, 2, 3 and 4.

Note that this feature is not supported by USART 0.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>