[LPC11U68] Fix GPIO init for specific pins

* Add GPIO initialization value for P0_12, P0_13 and P0_14 since default
mode for them are other than GPIO
* This issue reported here:
https://developer.mbed.org/questions/4874/Using-SPI-on-LPCXpresso11U68/
Merge pull request #679 from toyowata/master

Targets: LPC11U68 - Fix GPIO init for specific pins