board: lpcxpresso54114: Rename to lpcxpresso54114_m4

As part of the prep to support the M0+ core on lpcxpresso54114, we
rename the lpcxpresso54114 board port to lpcxpresso54114_m4 to be
explicit.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>