boards: arm: add support for lpcxpresso11u68 board

This patch adds support for the LPC11U68 LPCXpresso board based on the
LPC11U68 SoC.

Signed-off-by: Maxime Bittan <maxime.bittan@seagate.com>
Signed-off-by: Simon Guinot <simon.guinot@seagate.com>